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.  Predation is a serious issue with Koi ponds, especially during the overnight hours.  Cats, raccoons, 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.
  2. Parasitic infestations of 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.  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. 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.
  4. Low oxygen levels can certainly be one of the leading causes of the death of Koi fish.  Unfortunately, many people do not realize that they are having this issue until it is too late.  Adding plants or a small fountain or waterfall to your pond should create sufficient oxygen and flow to keep your fish alive and happy.
  5. High pH levels and high bacteria levels can all 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.

26 Responses to Top 5 Most Common Koi Pond Problems and Their Solutions

  1. Kim says:

    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 says:

      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 says:

    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 says:

      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 says:

    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 says:

      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 says:

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

    • Koi-Care staff says:

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

  5. Frank says:

    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 says:

      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 says:

    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 says:

      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 says:

    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 says:

      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 says:

    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 says:

      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 says:

    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 says:

      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 says:

    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 says:

      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 says:

    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 says:

      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 says:

    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 says:

      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 says:

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

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