Top 5 Most Common Koi Pond Problems and Their Solutions

Typical koi pond

Having a Koi pond can not only increase the visual appeal of a property, but it can also aid in developing a passion for an environment that you are able to maintain for yourself.  Koi ponds can certainly encounter a number of issues throughout time, especially considering the amount of time and effort that one puts into maintenance, but this does not mean that such problems cannot be solved and alleviated with a few simple suggestions.  The following will outline 5 of the most common problems associated with Koi ponds, as well as ways in which such problems can be solved.

1. Water Quality

This is THE leading cause of the death of Koi fish.  It largely stems from the fish’s waste products and the first iteration of that is ammonia. Unfortunately, many people do not realize that they are having this issue until the koi’s gills are burned by the ammonia.  

You can use chemicals in the short term that sequester the ammonia but ultimately you really want to address your filtration and levels of beneficial bacteria.

In the biological cycle of a koi pond ammonia is produced from fish waste, which gets turned into nitrites, then nitrates.  Its important to become familiar with the basics of this process to understand better how to treat and prevent water quality issues.  This article and graphic describe this process in more detail. 

Really high or low pH’s can also contribute to the death of fish.  Koi ponds may have a pH range between 6.8 to 8.2 but 7.0 to 7.5 is really ideal.  There will most likely be natural daily swings of pH of a small amount but that is not something to be concerned about as long as they are small. 

The concern becomes big swings in a short time or generally pH levels that are outside the acceptable range causing koi skin to become vulnerable to bacterial infection.  Two main solutions to keeping pH levels in check are regular testing and if need be adjustments via buffers and acids.  In short, buffers will increase pH while the addition of acids will reduce pH.

2. Diseases

Parasites on or in your fish can not only be difficult to identify, but they can be nearly impossible to identify as many are too small for them to be seen by the naked eye.  

Bacteria are everywhere in nature, including your koi pond.  There are good bacteria and not-so-good bacteria.  If your fish are stressed for any reason that can increase their vulnerability to bacterial infections (especially if they have a lesion or injury).

Water quality is at the heart of disease prevention so although you can treat diseases as they arise you will be doing yourself and your fish a favor by getting your water chemistry right.  

If you are experiencing an issue of this nature, then consider a better filtration system such as one that incorporates a UV sterilizer. For more on koi diseases see this article.

3. Algae Growth

If you find that your Koi pond has excessive algal growth, which can decrease your ability to see your fish, then there are several ways to manage the algae in your pond. 

You can choose to either tint the pond water, which will reduce the ability of the algae to gather sunlight, or you can add just a slight amount of salt in the pond, but only if you do not have other plants growing that don’t have some salt tolerance.  

At the heart of algae problems is water quality/chemistry.  If you can remove one or more of the elements that allow algae to thrive you will come out on top.  

Algae Box-what algae needs to grow

4. Overcrowding

When it comes to koi, less is more.  You don’t necessarily get more satisfaction out of 30 fish than you would with 10.  Keeping your population to a manageable level avoids lots of other potential problems like excessive waste build-up, depletion of dissolved oxygen, stress from crowded conditions leading to disease.

The more koi you have, the more koi you will have.  Koi breeding will really inflate your population size. Crowded conditions will cause a physiological response in koi that “tells” their bodies to reduce growth hormones resulting in smaller koi overall.

5. Predation

It can be a serious issue with koi ponds, especially during the overnight hours.  Cats, raccoons, wading birds, river otters and even larger coyotes have been known to prey on the fish in Koi ponds. 

If you are finding this to be an issue for you and your fish friends, then consider making the water too deep for large birds or animals to stand in or place netting on the surface of the water to pose as a barrier between the fish and the predator.

In conclusion, taking care of your koi and the pond they live in is a process but one that you can learn and get better at over time.  Its easy to get overwhelmed by what may appear to be complex but any large problem is merely many small problems that can be tackled one by one.  By addressing water quality first and spending the most time on mastering that, fish health and pond health will follow.

By |2018-10-08T19:57:29+00:00June 13th, 2013|General Koi Information|100 Comments

About the Author:

I have been interested in fish for over 25 years. I have two degrees in marine science with a specialization in fish ecology and physiology (as well as a chemistry minor). Like many kids, I had a goldfish growing up but decades later took care of several koi ponds. Koi are such great pets and very accessible for many folks wanting to get into owning koi in a pond environment.


  1. Kim July 6, 2013 at 2:18 pm - Reply

    We are located in New England and have a large pond fed from natural wet lands. We rely on nature to filter the water. This has worked well for us and the 30 koi who share the pond with the local frogs and fish.

    For the first time in the 10 years we’ve had koi, I have noticed an irregular shaped bump on the back of one koi. This was first notice about two months ago, in May. I have not noticed any similar bumps on th other koi. The pond is too large to net tho koi for closer inspection. Can adding salt to the water help heal this fish? Will the salt cause any problems downstream? How can we determine the amount of salt that would be healthy for this environment?

    The water from the marsh can be very dark during the warmest days of summer. It occasionally has a film that looks like an oil slick. The fish dont seem to mind but we dont care for it. Would salt clear the water? Can you recommend some non invasive plants or natural additives which would be good for this open environment?

    • Koi-Care staff July 8, 2013 at 2:01 pm - Reply

      I would be cautious about adding salt as a whole-pond treatment. Though the koi can handle a certain amount of salt exposure for a limited time you risk affecting other plants and animals in the system. Really the best thing to do is to somehow lure your affected fish over (perhaps with koi chow) and net it up and quarantine it. You may have to eventually cull this fish as if won’t be worth losing your whole group for one fish. As for the oil slick I can’t say I know off the top of my head what that is but it clearly has something to do with the increased biological activity of the system associated with increasing temperatures. With respect to plants that would be ideal for your pond I would say water lillies are a great choice as well as Elodea (American water weed). Elodea is great for cover and for producing dissolved oxygen (DO) in the water (which is nice for hot days that can create a low DO scenario). Another cool thing is that even it gets uprooted, say by a rooting koi, it can still grow unattached to the sediment. Hope this helps.

  2. leong December 3, 2013 at 12:13 pm - Reply

    if the water in the pond green in colour, will it affect the fish. Reason for the water turning green is because antibiotic was added because the fish was not properly transfered from another pond.

    • Koi-Care staff December 5, 2013 at 3:46 am - Reply

      I would say that your fish should be fine even though the water has turned a green color, especially since its because of the antibiotic. What kind of antibiotic was added?

  3. tony May 2, 2014 at 11:46 pm - Reply

    We are located in California and we have a 500 gallon pond with 4 koi fish that are each about 10 inches long and around 6 goldfish that are 1-2 inches long. We have a two foot wide water fall connected to a 3600 gal/hr pump and a large skimmer basket. We built the pond about 2 months ago, but we still have problems with green water/algae. I was wondering if you have any suggestions on how to clear the water. We have 4 large plants and two water lily clusters.

    • Koi-Care staff May 6, 2014 at 12:55 am - Reply

      Green water which is essentially single celled algae is often associated with spring and summer as that is when ponds begin to experience increases in sunlight and water temps.- both important elements for algal growth. However, another element that aids in algal growth is nutrients such as nitrates. So right off the bat check your water chemistry and make sure you’re not feeding to excess which can result in higher nitrate levels. There are chemicals out there as you’ve discovered that will help to mitigate for green water and those include a products like AlgaeFix and Accu Clear. To really get at the heart of the problem though your water chemistry comes first and after that the next best approach is an in-line UV light as part of your filtration system. UV is a very effective control for algae and doesn’t involve chemicals that have the potential to complicate things.
      Hope this helps.
      Good luck

  4. M Wilson May 8, 2014 at 10:26 pm - Reply

    My fish seem to be digging up the bottom of our pond. Water is always dirty now. Solutions?.

    • Koi-Care staff May 22, 2014 at 3:49 pm - Reply

      perhaps an addition of gravel to the bottom may help keep unconsolidated mud from becoming suspended.

  5. Frank May 21, 2014 at 8:31 pm - Reply

    I can’t seem to get an algae infestation cleared up. The pond was just power washed 3 weeks ago. Within a few days algae growth began. Now the pond liner is covered with string algae up to 12 inches long. I have added plants in the waterfall stream and 4 lilies , 2 giant marigold… The pond is in an open area and gets sun all day.

    • Koi-Care staff May 22, 2014 at 3:46 pm - Reply

      String algae won’t do well unless the conditions are right and that means more than just plenty of sunlight. You most likely have the proper nutrients in your pond to spur the growth of this algae. The usual suspects are phosphate and nitrates. Excessive nitrates are the result of nitrogenous waste from your fish and any uneaten food will yield phosphates. The problem is you can use something like Algae Fix to effectively kill the algae but you will still have to remove it anyway b/c once it dies and begins to decay and that will result in an eventual increase in nitrites and nitrates( So test your water and see where your levels are ( and begin to deal with your water chemistry then consider the plant coverage in your pond. Lillies are great for providing cover and essentially keeping your pond cooler and inhibiting as much sunlight penetration. Try for 60-70 percent surface coverage. I know you just planted some additional plants but try to hit that coverage goal. I think once your water chemistry to brought back to the proper levels I would just remove the string algae manually (in the beginning) rather than going right for the Algae Fix.
      Good Luck

  6. audrey June 2, 2014 at 1:27 pm - Reply

    I live in Central VA and have about 15 koi in a 1000gal pond. Yesterday the fish (all of them) were frolicking around like crazy all day, chasing each other all around. This morning 2 are dead and the rest are very lethargic. I don’t know what’s going on. Could they have stirred up stuff in the bottom of the pond and made the water toxix? I haven’t been able to find anything on the web addressing these symptoms.

    • Koi-Care staff June 15, 2014 at 1:29 am - Reply

      yes, this is possible. Toxic gasses can be released from sediments and muck built up over time but it typically occurs if someone walks through it quite a bit or the muck is mechanically dredged. Spring and summer are times when koi will breed so I am thinking that what you had was some mortalities due to breeding competition between males and/or an excess of males on the female(s). A ratio of two males to one female is recommended -anything more may cause problems. A full moon will also help to initiate mating but an approaching storm with its associated low pressure will also spark mating as well -do you remember if had a storm around the time they were chasing each other around?

  7. Heather Smith June 27, 2014 at 4:29 pm - Reply

    I live in RI, my pond is about 7000gal. I have two bio filter pumps with waterfalls, two UV lights and I have been using a lot of beneficial bacteria, barley and algae fix. My water is olive green, I cant see the fish, and there is a constant accumulation of a thick olive green sludge that is all over the lining and several inches thick on the bottom. I have vacuumed it out several times, we have done a 2000gal water exchange, and I recently added an aerator. But the olive green sludge just keeps coming. I’m desperate!! Any ideas?

    • Koi-Care staff June 30, 2014 at 5:50 pm - Reply

      having the UV is great but they may be getting weak (towards the end of their life) or the glass of the light may be obscured in some way by particulates. It sounds like you are taking all the right steps. One thing you may want to try though is koi clay. Its totally natural and you may just find that its the silver bullet to your green water problems. Obviously you will want to keep an eye on your water chemistry too -if that is too high in nutrients its just fueling your algal growth.

  8. Lora K June 30, 2014 at 12:32 pm - Reply

    I have had a KOI pond for four years when I bought a house. Never had an issue, the pond is in an atrium that gets sunshine but some of the outside elements aren’t able to get in. No issue until about a week ago with green algae. I treated the water and it has cleared up but now I have lost on of my fish and the other two don’t look so good. At a loss to be honest.

    • Koi-Care staff June 30, 2014 at 5:49 pm - Reply

      t sounds like the green algae is an indication of a larger problem and that problem most likely lies with your water chemistry. You can’t have algae doing well with just sunshine or just nutrients -they need to have both of those elements in the equation to thrive. My guess is that your filtration is not as effective as it used to be and not removing those excess nutrients that are allowing the algae to do well. So two things: first I would look into UV sterilizers to add to your filtration system (its generally a good idea to have that in play especially if your algae is the free floating, green water type) and second I would look into giving your filtration media a boost with a “microbe-lift” or something similar. Make sure you’re not overfeeding either or that something isn’t dead on the bottom (or anything obvious like that).

  9. Fred August 2, 2014 at 3:23 am - Reply

    When the waterfall in our koi pond is turned on there is an awful odor; I want to say methane but I’m not sure about that. Also, the water is very green even though the pond was recently cleaned. Unfortunately, the pond is direct sunlight most of the day. We do have plants in the pond and about 9 koi in a 1200 gallon pond.

    Thanks for any help you can give.


    • Koi-Care staff August 5, 2014 at 6:18 pm - Reply

      The smell from your water sounds like a sulfur smell that is typically associated with well water-is that the source of your pond water? The other thing it could be and this may be a long shot is that the action of the falling water reaching the bottom of your pond (not sure how deep it is where your fall empties) is stirring up anoxic mud which will also have a sulfur/low tide smell.
      As far was the green water goes there’s lots of good solutions for that but the best thing would be to see my latest article on dealing with algae. I can tell you though that a big asset in dealing free floating algae like what you have is a UV sterilizer.
      Good luck, grant

  10. Brenda Pretorius November 4, 2014 at 7:12 am - Reply

    May pond is always turning green can a put a sun block shade offer the pond to proven turning green. I do have the uv light and filters on the pond. This morning one off may koi’s is swimming on his side what can that be? I am living in South Africa

    • Koi-Care staff November 4, 2014 at 3:46 pm - Reply

      There are many things you can do to deal with green water. I will direct you to my recent article on this subject but in the meantime yes, you can put a shade over the pond, you can add a dye to the water that will block a lot of light penetration into the water and prevent the free floating algae from receiving light. You can add something called “koi clay” that will help with green water and your koi will love it. You need to check your UV sterilizer -is it dirty? is it big enough for your pond size? is the bulb getting older and less effective? Is your filtration sufficient for your pond size? Remember that as fish grow or as you add fish you add more demands on your filter system so what worked 3 years ago may not be sufficient now.
      A koi swimming on its side could be a lot of things -may be an air bladder issue. Its hard to tell at this point. Possibly an internal infection.

  11. Randy Kanten December 14, 2014 at 9:55 pm - Reply

    I have a 10,000 gal. pond.
    In it I have 4 – 10lb koi and about 30 various size gold fish.
    Last spring when I moved them outside for the summer they were fat and healthy. By mid-summer one of them always stayed in the shallows amongst the water lilies. It almost never came out for feedings. When I brought them in for the winter this one was very shinny and had the appearance of a Sturgeon. Is it possible it could have a tape worm? what could be wrong with it and how do I treat it? I am afraid I might loose this 6 year old Koi

    • Koi-Care staff December 22, 2014 at 4:55 pm - Reply

      Randy, well the shiny description is usually a product of carp pox which wouldn’t typically be fatal or cause the sturgeon look (which i am assuming is a thin, gaunt look) So if your fish is losing weight consistently then I would tend to conclude that it is “skinny disease” which is bacterial in origin. Here is an excerpt from my site:

      Treatment: Adding extra food to the fish’s diet can usually clear up this disease. However, sometimes this doesn’t help and if the bacterial infection persists, adding erythromycin to the fish’s food normally clears the infection up quickly.

      You will want to quarantine this fish no matter what you do as its hard to treat a specific fish when its mixed in with the others and is withdrawn. You may want to do a light salt treatment in quarantine to allow the fish a little osmotic relief while it fights the infection.

  12. Stuart Hunter January 16, 2015 at 5:31 pm - Reply

    I have plans to renovate my pond this coming summer so I moved the Koi into a large tank. The Koi have become quite aggressive when I come into the room feed them, so much so that the thermometer I put in the tank has been broken. Not a big deal in itself but it appears that one (or more) of the fish may have eaten parts of the thermometer. the parts are big enough that I don’t believe the fish will pass them. Is there something I can\should be doing? The fish appear to be healthy.

    • Koi-Care staff January 20, 2015 at 3:12 pm - Reply

      If this issue is of concern to you you can do a gastric lavage but you need the right sized tubes and such and a steady flow of water from say a hose.

  13. Michael January 24, 2015 at 3:17 am - Reply

    Im interested in owning Koi.. but i don’t have a place for a pond. Im wondering if Koi do well in aquariums?

    • Koi-Care staff February 8, 2015 at 3:28 am - Reply

      Sure, plenty of people keep small koi in aquaria.

  14. Jim April 5, 2015 at 4:35 pm - Reply

    Please help, I have large koi in my pond around 18in
    Long and weighing around 3-31/2 pounds, I’ve already lost one
    and possibly going too lose another.
    The fish seems to roll onto its side as if dying , then speeds of up / across
    The pond as if full of life?
    I’ve tried bacterial medication, and fungal medication,
    I must confess I’m no pond expert and wondering if I’m doing something wrong.
    My ponds 21ft- 6ft well circulated water and two waterfalls,
    2-5ft depth holding 800 imp gallons. Please, please help.
    Jim Glasgow Scotland.

    • Koi-Care staff April 11, 2015 at 2:46 am - Reply

      Hi Jim,
      First things first I would isolate your fish in a quarantine tank to be able to treat better (you may also want to put a heater in the quarantine and raise the temp up slightly). Secondly, one thing that comes to mind in terms of causes is costia so you may want to do a scrape for that,. Second thing that comes to mind is water quality. As this is Spring in Glasgow, you will often experience a bad combination of immune-depressed fish coming out of winter mixed with lower levels of beneficial bacteria in your filter media which results in the water quality going down and allows things like costia and other parasites to thrive. So test your water and of course look for high ammonia and nitrites. Do water changes as necessary and boost your filter media with beneficial bacteria (something like Microbe-Lift PL).
      hope this helps, grant

  15. Todd April 13, 2015 at 11:27 pm - Reply

    Came home this evening and the pond had a very strong fish smell. One dead KOI the rest seem to be fine. Just bought the place towards the end of last year. Have a local pond company coming this Friday to empty and clean the pond. Called and left a message with them tonight but was hoping you could answer this tonight to see if there is something I should do to save the rest of the KOI.

    • Koi-Care staff April 20, 2015 at 7:43 pm - Reply

      That’s tough to say what happened to your dead koi but the first thing I would do is check water chemistry- most problems stem from water issues. As it is spring you will see immune compromised fish having issues with poor water quality as they come out of winter dormancy.

  16. Sharon July 12, 2015 at 1:24 am - Reply

    This is my first koa pond, I started it about two month ago. I came home tonight and one of my fish had died. Imy wondering if I have caused this when I put the Algae Fix in the pond. Would you advise me to take out the other fish until I figure out the problem?

    • Koi-Care staff July 13, 2015 at 3:09 pm - Reply

      Algae fix shouldn’t be the culprit but yes, its a good idea to quarantine your remaining fish until you can properly test the water etc.

    • Michael July 14, 2015 at 10:51 pm - Reply

      I also had a small 120 gallon water feature with some plants and small koi and goldfish. About 3 weeks in the water was getting rather green even with a fountain and bubbler. I added Algae fix and:
      Day 1: Dead goldfish
      Day 2: Dead Koi

      What did I do wrong?

      • Koi-Care staff July 29, 2015 at 5:42 pm - Reply

        Green water algae won’t grow without the right conditions like high Ammonia and nitrites so I suspect your water chemistry is out of line. Algaefix might kill the algae but the water chemistry issues still persist. That’s where I would start if I were you.

  17. Koi love July 14, 2015 at 3:32 am - Reply

    We are having huge issues with our 125 gallon pond. It is fairly new, about 2 and a half months and we are in south Texas. Over the course of 2 weeks, we have had many problems with our koi and goldfish. We have 6 small medium sized koi and goldfish ..I do regular water changes, about half to 3/4 and have used Prime and Stress Enzyme each time. Over the few weeks algae has become a major issue. I noticed that the koi would hide in their fish home at the bottom of the pond whenever I would do an algaefix treatment. They wouldnt come up to eat at times. We went away for a weekend vacation and had my mom come feed once a day and when we came back the water was almost like a green milk, but 2 days prior we had gotten the water pretty clear, as clear as it could be considering its summer. I did a 3/4 water change, did stress treatment and algae and had 1 koi start swimming sideways, died later. 5 goldfish also died the same way. Little did I know the chlorine was at toxic levels because I didnt put enough Prime in. We did another water change with Prime and the rest of the fish made it through. 3 days later we went on vacation for 5 days, just came back last night. Water did the same thing, green milk appearance, even though my mom was doing algaefix and routine 6 in 1 strip testing. I did a water change last night and treated the water with prime, stress enzyme and algefix. This morning the fish refused to eat.. they were all confined to the bottom of the pond. I checked the water, everything seemed fine except a small increase in chlorine and the water was still murky even after the water change. I did one more change, same treatments, this morning. I also placed a few lava rocks in our pond filter box. Hours later our smallest goldfish died and all of our 6 koi started coming up gasping for air. I shoukd note that We have a fountain and a pond pump that does 200 gallon per hour. Well, I started researching and assumed that this koi behavior was an oxygen problem, so we bought an aerator. An hour after running it, koi were still having issues and a water test showed very high alkalinity. Had one of our oldest koi start swimming sideways and die. 2 have been frequenting the top for the past few hours.

    How did our pond go from being normal, no deaths, for weeks and weeks, with the same style water changes to now koi dying and goldfish. What am I doing wrong? We are new to ponding but trying to figure it out.

    • Koi-Care staff July 29, 2015 at 5:42 pm - Reply

      1) when you do massive water changes you run the risk of major pH swings and that can cause problems.
      2) green water appearance is free floating algae which means you have conditions that support algal growth such as high ammonia and nitrites. Suggest a UV filter and/or giving your filter media a boost with Microbe-Lift PL
      3) south texas is hot -what kind of water temps are you seeing? Fish that are already suffering from say chlorinated city water and then you stack on top of that low dissolved oxygen and higher water temps it magnifies whatever problems the koi were having.
      4) its probably worth a read ( my article on algae in koi ponds should help your situation.
      5) your high alkalinity may be attributed to multiple things but the big water changes are most likely one of them.
      As you are prob. figuring out, most problems usually originate from water chemistry/quality so if you can navigate your way through that you will be able to handle most issues that come up.

  18. Thomas Rodrigues September 11, 2015 at 2:05 pm - Reply

    A client wants a Koi pond and the tempratures vary in India from 12 to 40 degrees Centigrade. The pond is 50m long and 3m wide and 1.2m deep, loosing water. They want a liner with rock lining and pebbles.

    1.Please advise if a cascade at 10m intervals would harm the Koi.

    2.What are the precautions for keeping the koi safe from such harsh temperatures.

    3. what would be chances of survival of the Koi in higher tempretures.

    Thanks and best regards

    Thomas Rodrigues

    • Koi-Care staff October 9, 2015 at 3:32 pm - Reply


      Please advise if a cascade at 10m intervals would harm the Koi.

      I interpret “cascades” as “waterfalls” and if so that interval would be just fine- it should inject a good amount of dissolved oxygen into the system and maintain some good flow.


      What are the precautions for keeping the koi safe from such harsh temperatures?

      40 degrees C is pretty hot and I don’t foresee your koi doing that well in those kinds of temperatures. You really don’t want to exceed 29-30. One thing you can do is install shade (ber that over the pond itself or install shade bearing aquatic plants) and another thing you can do is install chillers. I’ve never had to deal wit chillers but I know that there are companies that make them for that purpose.

  19. Christine Hinderliter October 5, 2015 at 3:11 pm - Reply

    My husband and I have noticed a large increase in algae in our pond- maybe about 500 gal- 1000 gal pond. We are at a loss as to what to do to stop this from happening. We have now noticed a serious decrease in the activity of our fish, I haven’t seen them in a few days, haven’t even seen them come up for food. we did have some inclement weather and the temperature around here has begin to drop. I was hoping that they are “hibernating” but because of the algae I cannot see them so I am not sure… any help or advice you can offer is great.

  20. Confused koi from Portland, Or October 22, 2015 at 9:04 pm - Reply

    I just started a new pond this spring 5000+ gallons. It has all the filters and has been tested over and over for nitrates ammonia ph alkalinity and phosphates. All are nil or in range respectively. So in theory perfect water conditions. Although only reached 50 F even though temps were consistently over 90 F. I ran it for a few months before I put fish in it and to get the plants up and ready testing it the whole time. The plants (even the hyacinth) all died before I could even put fish in it got plants from separate reputable clean sources. I have taken the plants out and put them in a green house and they have perked right up. I tried to put 6 fish in following all the right procedures and all 6 died the next day. Not due to any predators. So how can I test for chemicals that are not tested in the pond spectrum? Also it may be in my public water supply. How do I filter my tap from unknown chemicals? Or is there something I am missing?

    • Koi-Care staff December 1, 2015 at 3:04 am - Reply

      That’s strange that your pond never got over 50F even though it was hot outside -that to me suggests that it is spring fed (but I guess not if you fill it with a public water supply. This issue is obviously outside the realm of most koi pond issues as it deals with chemistries beyond the usual suspects.

      There is no chlorine whatsoever. Sorry forgot to ad that

      Okay, then you have obviously have something beyond the scope of what is typically tested in koi ponds. It might be worth contacting someone at the nearest university (environmental chemistry dept.) Or local government group who regularly tests local lakes and streams. They will have the equipment to do so.

  21. tom farooqi October 26, 2015 at 11:52 am - Reply


    I have 3 Koi and 5 goldfish in my pond, we live in Birmingham where its cold my small yellow goldfish seems to be bleeding and two koi keep swimming at the surface all day in circles and the other koi and 2 goldfish and 2 shubunkins are hiding at the bottom and come out some times

    • Koi-Care staff December 1, 2015 at 2:47 am - Reply

      if you have algae build up that usually means your water contains the fuel to allow the algae to bloom. In this case that fuel would be Ammonia and nitrites etc. So I would definitely get that tested first but yes, those plants could have brought in something unwanted.

  22. mike November 14, 2015 at 6:56 pm - Reply

    Finding my koi fish outside the pond. All have looked fat and healthy. What’s happening with our fish? Why don’t they stay in the pond?

    • Koi-Care staff December 1, 2015 at 2:49 am - Reply

      I’ve not heard of that happening too often. What other fish do you have in there? Maybe they are scaring them out? Perhaps otters are getting in there? Any signs of external parasites? That may cause them to jump in order to knock parasites off and are inadvertently ending up on the shore.

  23. Brian Thomson November 16, 2015 at 12:43 am - Reply

    Hi, I am quite worried about one of my Koi I have had two Koi for many years now, they are both quite large probably around two pounds. they live in an outdoor above ground level tank along with three goldfish, they have all lived there happily for over ten years with no problems, I have a water filter with an ultra violet light tube fitted which keeps the water reasonably clear in summer and also various plants, I do nothing apart from topping up the tank water in a dry summer, I have tested the water and it has all normal readings, I do nothing more than feed them regularly, however one of the Koi has started to keep it’s tail above the water surface, apart from when it’s feeding which it still does well enough but then when finished it always returns to this position, it’s tail appears to be clean and looks no different from the rest of it’s body with no injury or any other visible problem, night or day the fish adopts this position with it’s tail above the surface, he is not entirely in a horizontal position but stays at a downward angle, my other Koi behaves normally. Could my fish be getting old and simply feels better in this position or can anyone identify this behavior with a disease or some other malady? I would be very grateful for any advice.

    • Koi-Care staff December 1, 2015 at 2:49 am - Reply

      My guess would be a swim bladder issue. There is most likely pressure on the swim bladder from a food compaction impeding the normal operation of the swim bladder for regulating proper position in the water column. I would thaw some peas and feed them to the fish -this should penetrate the compaction caused by overeating on your koi’s part.

  24. Sad December 1, 2015 at 7:58 am - Reply

    Hi. I was house sitting my landlords house, and she had a beautiful koi pond that she had for over 40 years… never any problems.
    The first day she was gone, everything was fine. I get up the next morning to feed the fish and they are all dead. I am heartbroken and feel so responsible!
    We don’t know what could have caused this! Any ideas? Thank you so much!

    • Koi-Care staff December 30, 2015 at 8:32 pm - Reply

      well, disease usually takes time but one thing that can kill fish pretty quickly is some sort of chemical. A lot of times what happens is that an untrained person adds water to a pond using city water that has enough chlorine to kill the fish. Sometimes its runoff from perhaps an insecticide that was sprayed nearby. Did it rain the night before or was there lighting?

  25. Kayla December 8, 2015 at 2:09 am - Reply

    I have 5 koi fish. One of them has become very large and I think it is carrying eggs. My koi at the moment are breeding, but they are ignoring the large one. The males go after the other female in the tank. I’m getting worried that my koi will die from infection from holding her eggs. What do I do?

    • Koi-Care staff December 30, 2015 at 8:30 pm - Reply

      But you’ll want the egg carrying female to be with the males when she releases eggs or else they won’t get fertilized. Infection won’t be an issue, in the case of fish reproduction any eggs not expelled will get resorbed.

  26. Cem December 29, 2015 at 12:16 am - Reply

    Hi i’m quite worried about my Koi pond it is 6ft it’s Placed in decking I have very worried about my koi I have lost numerous of fish in the past 3 to 4 weeks I have 2 fish left I have tried cleaning out the filters I have even taken my water to be tested and he is telling me that it could be possible be that they don’t like the weather change or A bacterial infection in the pond NEED HELP BADLY Thanks

    • Koi-Care staff December 30, 2015 at 8:19 pm - Reply

      What are your koi’s symptoms? How cold is the water? Are they bloated?

  27. Wong Che meng January 27, 2016 at 8:50 am - Reply

    Dear Expert,

    I have 10 koi in my pond, all are Ok except one that is 5 years old, it cannot swim straight, its body is curve , thus moving in circles, before that the body is straight and swim straight. Now it swim side ways in circles, the fish stomach is very full.
    It feeds with its tail up, looks like it has lost balance and is lethargic . I feed vitamins powder directly thru its mouth, it than has energy to swim, but body is still curve.
    What could be the cause and what can I do to solved this problem. Appreciate ur advise.
    The Koi is about 22 inches length and could weigh about 2 to 3 kilos.

    • Koi-Care staff March 28, 2016 at 7:41 pm - Reply

      It could be one of a few things going on. You may have an swim bladder infection (or otherwise not functioning properly) such that the fish cannot inflate/deflate the swim bladder properly. There may be some compacted food in its digestive system and may need only feed to feed the fish some frozen peas. It may also be some kind of internal infection. Try the peas first though.

  28. Kayleigh February 9, 2016 at 12:37 am - Reply

    Heck of a job there, it absolutely helps me out.

  29. Cynthia February 18, 2016 at 10:43 pm - Reply

    I have 1 koi out 6 that has been laying on its side periodically and now has fluffy white stuff on its fins and tail. Water tests are normal, ph 7.5 water temp is 30 what is wrong with him?

    • Koi-Care staff March 28, 2016 at 7:36 pm - Reply

      I believe your koi has Epistylis. It’s a parasite and I recommend a salting regime. 2.5 lbs of salt per 100 gals. For two weeks. Divide the total amount that you are going to add and split it into 3 parts and introduce it over 3 days to ease them into it. You can also do potassium permanganate but salt is effective and less harsh. You might as well treat the pond rather than quarantining because the rest will eventually get it. It stems from a water quality issue though so you will need to address that after you clear this up

  30. Kate March 15, 2016 at 1:06 am - Reply

    We have a koi and about 8 smaller goldfish. My husband mistakenly put bait fish in the pond and they bred. How can I remove the bait fish without hurting the one koi and 8 gold fish? My husband called the bait fish brooder… Could we put a more agressive fish in there that will kill the bait fish? I am lost for solutions!

    • Koi-Care staff March 28, 2016 at 7:27 pm - Reply

      Fish trap.
      Drop it in there with some bait and when you pull it out sort through what to keep and what not to keep.

  31. Anna May 8, 2016 at 8:43 am - Reply

    Please HELP!!! We bought a house in WA one year ago that has a 3,500 pond and there is about 25 big koi fish there. When we bought the house the water was really nice clean but 2 months after the water become so green and we could not see our fish so we hired a professional to come clean our pond but 2 weeks after the water was even worse and for one year we have been doing so many water changes buying so many chemechels and are fish are sick and dying please please help

    • Koi-Care staff July 11, 2016 at 3:28 pm - Reply

      UV is always a good idea for koi ponds as it not only handles free floating algae like what you have but it also kills bacteria and fungus and viruses too. Additionally, Ionizers are also really great assets to have working for your pond as they deal nicely with free floating algae but they are not cheap -you can read about them here. It isn’t necessary to have an ionizer though so for the meantime you can use a product called Accu-Clear. It is a flocculator and essentially grabs and clumps free floating algae then brings it to the bottom where your bottom drain takes it to the filter. Its essential that the clumped algae gets removed from the pond though or the cycle will start anew. It would also help to ramp up your beneficial bacteria colonies with something like Microbe-Lift. Ultimately your water chemistry needs to be at levels that don’t facilitate green water algae in the first place so that needs to be analyzed right off the bat.
      Hope this helps.

  32. Christie May 29, 2016 at 12:00 am - Reply

    We bought a house that had a koi pond already. It didn’t have a pump of any kind and it was pure black, but the fish were alive. We hired someone to come out and clean it, replace the liner and re-fill it. The next morning we woke up and the water level was down a bit, but not too much so we didn’t bother with it. That afternoon when we got home from work, half of the water was gone. I called him and he told me to turn the pump off and fill it back up, that it could just be splash out from the pump. So, we did that and the next morning, it was low. Lower than the morning before, but higher than the afternoon so we topped it off and went to work. That afternoon it was low again, so he came out and worked on it. It turns out that the liner had slipped and the pond was leaking. There has been no change in the water level since then(yesterday). Today the fish are dying, we are not able to get in touch with the pond guy this holiday weekend and are worried about out the ones that aren’t dying yet. What can we do?

    • Koi-Care staff July 11, 2016 at 3:05 pm - Reply

      My guess is that your dying fish has something to do with a rapid fluctuation in pH which can happen with large water changes. What is you water chemistry like?

  33. Phil Beezhold June 8, 2016 at 7:41 pm - Reply

    I have a pond that is 6 ft deep, and about 12 ft across. I have a lot of koi fish but they have been fine. Recently, they have been dying off, all of them have been thoroughly inactive resting mainly at the bottom of the pond, but occasionally the largest are jumping out of the pond. They appear to have a white film on them, but I cannot tell if it is more than what they normally should have. Why are they dying? And how can I help them to not die?

    • Koi-Care staff July 11, 2016 at 2:52 pm - Reply

      Two things come to mind (and I am inserting sections from my disease article on my site)
      1) One of the easiest protozoan parasites to see under a microscope, and subsequently confirm your fish is infected, is Trichodina. An infection of this parasite can be detected by a gray-white opaque appearance on the body of infected Koi. Trichodina is a warm water parasite and can survive in the water for a considerable amount of time without a host. Visually, they are perfectly round with hundreds of little hooks that look like cilia. It rotates continuously as it moves through mucus, causing damage to the Koi’s tissue. This parasite attacks both the skin and gills of your Koi. Infected fish also often show symptoms such as flashing, rubbing and lethargy.
      Treating Trichodina: Treat this disease with a five day course of increased salinity (0.5 to 0.6%). Due to increasing tolerance of some organisms to salt treatments a course of formalin may be necessary.
      Columnaris: This bacteria will attack sites of injury but will cause fin, tail and mouth rot. Additionally fish can be vulnerable to it during times of stress. Your fish may develop a white film on their skin and display sunken in eyes. It can be a rapid killer so be sure to take swift action.

      Treating Columnaris: Feeding your koi Aquatic Nutrition or Debride RX will definitely help your fish with an “inside out” treatment strategy.

      Ultimately though these things usually have their root in water quality though sometimes pond owners will introduce a new fish in the pond that hasn’t been properly quarantined and disease can be brought in.

  34. Chad June 25, 2016 at 6:55 pm - Reply


    I had 5 koi, 2 goldfish and 2 oranda goldfish. I Cleaned my pond one day because there was a lot of algae and I could not see anything. I took out all the water and scrubbed the walls of my pond to get all the algae off. I added new water and put the aqua safe liquid in to make sure it was safe for my fish. After one week I had 2 of my koi jump out of my pond and then die within a couple of days later. They did not have anything left of their tail. Then a week later I found another koi dead with its tail also eaten. So today I decided to clean my pond again and found another koi dead! These koi were smaller than the goldfish so I was assuming that the goldfish were attacking the koi? Or is there something I am missing. Also, these goldfish that I got were from my uncle who has a koi/goldfish pond and they reproduced in his pond so he thinks they are half goldfish and half koi. I dont know if that makes a difference but anyways Please help!

    • Koi-Care staff July 1, 2016 at 7:24 pm - Reply

      doing massive water changes like that can throw your pH way off. Big swings in pH can harm your fish. Were you testing the water throughout the process?

  35. Monica June 29, 2016 at 12:09 am - Reply

    I have heard that UV is bad for ponds. Is this true because I have pea soup green water and was thinking about getting one to help with this problem.

    • Koi-Care staff July 1, 2016 at 7:19 pm - Reply

      Certainly not. Direct UV on a koi or any living creature is bad but with koi pond UV sterilizer systems they are contained and kill only the bacteria, viruses, protozoa and free floating algae that passes by its light. Best of all, there is no chemical residue to worry about.
      Here is a link to my filtration article which talks about, in part, UV.

  36. Anthony Croft June 29, 2016 at 12:18 pm - Reply

    Hi have read previous comments and think my fish may have costia, so will be treating again for that, but whenever i clean out the pond filters they have an abundance of red bloodworms, are they a good thing ? as i assume they will eat all the organic matter inc fish poo and un eaten food. Or should i try and get rid of them, i recently built a bigger pond and accidently overstocked it with 12″ to 18″ koi, i have at least double the filters required and about 90w 0f uv an electronic blanket weed killer and 2 air pumps for when the 3ft drop waterfall is off during the night, i have been losing fish regularly since having the new fish, i have treated the pond with salt as well as anti parasite and anti fungus treatments more than once over the last 2 months, am still losing koi, found a dead 18″ koi this morning but was expecting it as it was very lethargic last few days as well could see some scales sticking out on the rear 1/4 of the body and would not move at all, apart from it being dead, could not see anything obvious apart from a reddening of the skin. the rest of the fish appear to look healthy. ive lost 12 so far from young 3″ koi to this 18″ koi, have not lost any goldfish or grass carp, just koi. All the koi have been lethargic before death apart from one 12″ koi which was a complete surprise as it looked normal and healthy. PS pond is under a pine tree and has a sun sail over it to keep sun and pine needles out. running 1x 10k and 1x 5k pump bottom drain fed multi chamber filter with vortex 1x 12k and 1x 8k pressure filters and just about to add a 20k gravity filter.

    • Koi-Care staff July 1, 2016 at 7:18 pm - Reply

      This is from my site on costia:
      Malachite green and 37% formalin can be used but you will need to make sure there is no salt in the pond to start with. If this treatment is chosen be sure to increase aeration.
      Not sure if you have tried the above but it might be worth it.
      The bloodworms are most likely midge fly larvae-don’t worry about those.
      What kind of numbers are you seeing when you test the pond for ammonia, nitrites, nitrates etc? It seems to me like something is off and allowing your pond to become afflicted by something doing a number on your koi (probably costia though if you saw one of your fish with scales raised up that sounds like dropsy).

  37. Edo320 June 29, 2016 at 12:58 pm - Reply

    I have indoor koi pond but it doesn’t have sun light, and many says sun light is great for koi color. What kind of uv light i need for that problem??
    I use UV light for filtration and it’s great btw.

    • Koi-Care staff July 1, 2016 at 7:17 pm - Reply

      I haven’t come across this issue before but i would expect you could use the same lighting the reptile folks use with their animals. They are called Sun Lamps.

  38. Carl August 18, 2016 at 7:32 pm - Reply

    Help, one of my koi has stopped swimming with the others, he looks like he has some dead scales on the right side, I’m worried because he is only using one fin.
    He is a ghost koi and spends most of his time at the bottom of the pond hiding which is fine, but While the others still seem to fly about he is just sitting on the bottom.

  39. Barbara September 14, 2016 at 3:38 am - Reply

    Also I may be somewhat overstocked. I dont have a count but about 25 small to med fish and Koi and about 6 larger size Koi, maybe 8 to 10″

    • Koi-Care staff August 8, 2018 at 1:19 pm - Reply

      To get a better understanding of your ponds capacity use my online calculator:

  40. Deidre Brown December 30, 2016 at 6:26 am - Reply

    I have lost 9 fish in 3 weeks. The once left have become very stationary in one corner, and dont rush for food. Nothing has changed, lots of oxygen, same food – the only thing was a big storm when the first few died. Its a 3500 lt pond with 9 fish left had 18 for 20 years!!!
    What has gone wrong????

    • Koi-Care staff February 3, 2017 at 8:31 pm - Reply

      hi, sorry for the delay in responding. For fish to die that quickly it usually means something went wrong with the water chemistry. You had a big storm? One that dropped a lot of rain? Sometimes when there is a lot of rain there is a lot of associated runoff from the land into surrounding ponds. It is conceivable that some pesticides could have been in the mix of that runoff that entered your pond. How did the dead fish appear? Wounds? sores? anything out of the ordinary?

  41. Dwight Jiro Gonzales March 29, 2017 at 4:26 am - Reply

    My koi fish is gathering in one side because I put some plants in my pond is that bad ?

    • Koi-Care staff August 8, 2018 at 1:07 pm - Reply

      that is strange -what kind of plant?

  42. Alan April 12, 2017 at 4:59 pm - Reply

    I recently added benifical bacteria to my pond filteraton system. It cleared the water but there is excessive sudsing. How do I eliminate the sudsing.

    • Koi-Care staff August 8, 2018 at 1:03 pm - Reply

      Alan, sometimes that effect can be related to brand. Was it Microbe-Lift? I’ve occasionally heard pond owners talk about this in the past (though its never happened to me)- it won’t be a problem for your water chemistry or fish and should clear up in a few days. I am curious though what is going on chemically or physically after the addition of beneficial bacteria to cause this -I think I will contact some manufacturers and see if they have a good explanation and get back to you.

      update: This is from Microbe-Lift

      Hi Grant:

      Normally foaming is from high phosphate.

      You may want to call Ecological Laboratories at 800-645-2976.

      Jeri Becker

  43. Aiden Chamberlain May 17, 2017 at 10:46 am - Reply

    Hi everyone can you help me out I have four koi that are gasping near my stream outlet and I cannot seem to tell what is wrong with them I have a mixture of koi, orfe, mirror carp and goldfish and four of them are gasping at my stream the pond water looks clear and i have a good steady flow going into the pond from the stream and also have an airation pump always on so enough oxygen please help someone

    • Koi-Care staff August 8, 2018 at 12:58 pm - Reply

      hi, so you definitely have enough aeration in your pond? what is the water temperature? How many fish total? (this will determine oxygen demand in your system). No ammonia in your water? High ammonia can burn fish gills and result in gasping at the surface.

  44. Judy Davis July 12, 2017 at 12:00 am - Reply

    Our Koi Pond is 30′ long and 15′ Wide it has 60 or more Koi. We have had the Pond for 16 years. We have had every Chemical Every Filter UV lights Charcoal Filters, plant Filters , rock Filters some work for a while nothing last. We have such a Sewer Sludge and Stink build up in the waterfall Barrels. We clean out everything weekly. Nothing Helps. What is wrong. Now we have a Water problem our well water is very low and we can’t waste what water we have by empting the Pond and filling it up we have a Vaccum but can’t use it no where to put the fish while cleaning out the Pond and refilling. We are perplexed. Do you have any suggestions?

    • Koi-Care staff August 8, 2018 at 12:49 pm - Reply

      That is a lot of fish and in my experience fewer fish means fewer problems. A lot of the times water quality issues can be linked back to overcrowding and overfeeding. How does your water chemistry look? Are you supplementing your filter media with beneficial bacteria to boost your pond’s ability to assimilate excess nutrients?

  45. darla July 24, 2017 at 5:46 pm - Reply

    We have had our pond maybe 5 to 6 months. We bought 10 small ones 4 med ones and 4 large ones. The algae states getting pretty bad so we bought some stuff that was suppose to help with that. I am thinking we put to much in and lost all but 5 (so sad) the ones left have been doing great so I bought 6 more small ones. When we started our pond we didn’t put a filter system in it … so yesterday we put a filter system in and a stream! We put one of our small pumps in the water just to have movement in the water. Within minutes all our fish were at the top of the water and looked like they gasping for air so we took the pump out. We have a waterfall on one side and a stream on the other side they have plenty of oxygen. We have lost 6 since yesterday what happened ???

    • Koi-Care staff August 8, 2018 at 12:45 pm - Reply

      My guess is that they are gasping due to high ammonia. Ammonia can burn the gills and render them less efficient. You may want to test your water chemistry.

  46. John August 23, 2017 at 1:19 am - Reply

    Can I buy a water treatment kit to test water levels for my koi pond.

    • Koi-Care staff August 8, 2018 at 12:41 pm - Reply

      Sure. Here’s one.

  47. Phil September 19, 2017 at 3:54 pm - Reply

    I have major problems with my koi they seem to be gasping for air but have plenty and some have gone a pink colour also a couple have white bits on them almost like fluff treated with medifin and then a parasite treatment with no luck around 15 have died in 3 weeks any ideas would be great

    • Koi-Care staff August 8, 2018 at 12:36 pm - Reply

      sorry for the late reply (been cleaning up hurricane debris). Your description sounds like ich which is pretty common. Here’s an excerpt from my site on that.
      One of the most common diseases found in Koi is Ich (aka; White Spot disease). It is a protozoan that begins its growth in the pond and later attaches to the gills of the Koi as it matures. The ich parasite initially appears like little white grains of salt on your Koi. Ich can kill smaller Koi, especially in crowded ponds and can cause other bacterial infections in your fish. It is usually borne out of poor water quality so to deal with the disease at its root, proper water chemistry will have to be achieved.

      Tropical fish specialists frequently deal with Ich. It can, however, also infect cold water fish. They must attach to the fish within 48 hours of hatching or they will die. Once they attach themselves to fish, they dig into the skin and feed on the fish’s tissue. After feeding on the fish for approximately three weeks, they detach themselves and move to the bottom of the pond to reproduce. The Ich hatch out of cysts at the bottom of the pond and use small hair-like tentacles called cilia to swim about.

      Treating Ich
      There are a couple of widely used methods to treat ich. The first one requires the koi owner to increase the salt concentration of the pond or quarantine tank to about 0.5% over a period of days. At the same time increase the temperature of the water gradually to the mid 80’s F while increasing aeration. This quarantine should last 2 weeks and is an effective and cheaper way to deal with the ich protozoan. The second method is a tried and true method for dealing with parasites in general and involves malachite green and formalin. Using both simultaneously is definitely going to be effective against ich but you may be able to simply treat with malachite green alone. For short treatments in quarantine, one could do a quantity of 1.5 mg of malachite green for every liter of water for up to 1 hour (or 6mg of malachite green for every gallon of water). Always wear gloves when handling both malachite green and formalin.


  48. Sheila March 28, 2018 at 7:33 am - Reply

    The previous owner reportedly painted the pond with some type non-pond specific paint and her fish died. This was 10 to 15 years ago. I want to use the pond but don’t know if I should try to remove the old paint first. I am not sure if I will just place floating flowers in the pond or try to add fish. There is a fountain in the pond also.

    • Koi-Care staff August 8, 2018 at 12:22 pm - Reply

      Yes, I would definitely do what you can to remove the old paint. The paint’s toxicity is most likely still able to leach into the water.

  49. Savannah May 22, 2018 at 12:49 pm - Reply

    We recently added a new Koi to our pond, a beautiful 8” black and red Koi we named Draco. From the moment we introduced it to the pond, the reigning king Smog, a black and gold Koi about 9”, has been on it.
    Smog separates Draco from the rest of the fish, keeping it pinned against the walls of the pond and whenever Draco tries to move Smog is right back there pushing it back against the liner, trapping it. Our new fish hasn’t eaten yet. And It’s not the first time that Smog has done this, showing what I’m taking as aggressive behaviour, however the other fish he did it to got snatched by a Herron.

    I’ve searched many sights and none can give me an answer as to why Smog is acting this way, how long it goes on for, can I do something to stop it? It’s been over a week and nothing has changed.
    We are worried about our new fish, and there is another Koi that a friend is asking if we can take off them because it is to large for their tank and our pond has a lot of space. But I’m worried to add another fish when the ones behaviours are so strange. What is going on? Please help!

    • Koi-Care staff August 8, 2018 at 12:21 pm - Reply

      Can I assume smog is the largest of all the koi present? this sounds like a dominance issue. You could try to see if it settles down and they learn to live with each other or removing one of them -thats kind of your only two options here. Its just one of those things. People that have several dogs don’t always have a good time of it. You may have certain individuals that don’t get along with another dog. Have you had fry after mating season?

  50. barry parsons June 30, 2018 at 8:33 pm - Reply

    Koi Carp, can someone help me with the carp i have in my pond , this is a natured pond and as been on the grounds for some years. The pond is about 75 by 75 yards I’ve never had the water tested, but the fish seamed to be ok, they have been in the pond for about 5 weeks but now they are dying. any one got any idea on what i should do next. This would be a grate help i want to put more fish in but need help.

    • Koi-Care staff August 8, 2018 at 12:19 pm - Reply

      So testing the water would be the first thing you’d want to do. There are so many variables involved that its tough to say for sure what the issue is. Its likely a water chemistry issue related to excess nutrients however it could be something unexpected like a recent rain washed some sort of insecticide or other harmful chemical into the pond.

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