Why Pond Algae Occurs and How to Get Rid Of It

string algae koi-care.com

String Algae

It is the case that every spring, as temperatures rise and daylight hours increase, an algal bloom occurs in almost everyone’s koi pond.  Dreaded algal blooms, like green water that make koi disappear from view or unsightly string algae that seems to pop up from nowhere, seem to be an inescapable fact of pond life.  Being able to effectively control pond algae is going to be crucial.

There is a lot that happens during the transition from winter to spring.  One thing that many koi owners experience is an increase in disease.  This increase is, in part, linked to the fact that little beasties like parasites are doing well in the increasing water temperatures but a koi’s immune system is still coming back into full strength so there is a period in the spring when they are more vulnerable.

Additionally, the increase in temperatures, sunlight and available nutrients from dead and decaying plant material and fish waste act to fuel algal growth and a bloom occurs.  But what about the beneficial bacteria in the filter media?  

They are still there but, like a koi’s immune system, are not at full strength yet (more on beneficial bacteria later).  Algae are pretty simple as living things go.  They need sunlight, carbon dioxide (given off from the gills of respiring koi and atmospheric CO2) and nutrients (nitrates, phosphates, ammonia  etc.). 

There is more to the story though when it comes to what algae need and how well they will do.  Algae do well when pH is on the higher side and this is because certain nutrients are more readily assimilated by the algae under these conditions. 

Algae also thrive under conditions of stagnant water or decreased water flow (you’ve probably noticed that stagnant ponds tend to harbor a lot of algae).  Different algae behave differently though- read on for the characteristics of the most problematic types.

Two main types of algae

1) Phytoplanktonic (free floating) which includes types that cause “green water” or “pea soup” water conditions. This type of algae may be the most common to afflict pond owners.  Spring is often when ponds turn into what appears to be a large vat of pea soup-certainly not how koi keepers want their ponds to look. 

Besides being a spring bloom occurrence, this single celled algae is often associated with newly established ponds as well due to the fact that the filter hasn’t had time to establish a sufficiently large bacterial population yet.


Green Water Algae

Green Water Algae


2) Benthic (attached) which includes “string algae” or “horsehair algae”, “water net” and “blanket weed”. String algae can be a tough one.  This algae can remain dormant for years in a dried state until introduced to water after which it will thrive. 

Another problem is that when you manually remove it from your pond (which is the best way) the action of removing it causes it to release reproductive spores into the water and the cycle starts again. 

As string algae tends to produce a good deal of dissolved oxygen it tends to aggregate bubbles tangled in its “hair” and before long a big, unsightly mat of the stuff floats to the surface further reducing the beauty of your pond. 

Of course, something that produces dissolved oxygen in your pond is a good thing, right?  Yes, up until the point that it dies, sinks to the bottom and is broken down by bacteria that use oxygen to do so thereby depleting your pond of dissolved oxygen.


string algae mat

String Algae Mat



Algae Box-what algae needs to grow


The following is a list of ways to prevent algal growth in your pond.


Sunlight is a big component that is necessary for algae to thrive so by shading your pond in some way you can effectively reduce some of the potential algae fuel entering your pond.  One way you can do this is the old fashioned way- trees.  Try planting trees that provide canopy overhead near your pond. 

Besides aerial shade there is also the aquatic kind.  Pond owners have, for a long time, installed aquatic plants like lilies in their pond to not only create shade but they make your pond more aesthetically appealing. 

The “magic” number to shoot for when it comes to aquatic plant coverage is 60-70% surface coverage.  Another way to reduce light penetration is through non-toxic coloring agents (dyes) that essentially tint your water a certain color and reduce the available light in your pond.  

This has become a “go-to” solution for plenty of pond owners.  They typically last a long time and you have a few dye colors to choose.  If you reduce the sunlight penetration you reduce algae’s ability to thrive!  Here are few good dye choices to check out.

Reduce Nutrient Loads

Nutrients like nitrates and phosphates are key to algal growth so by reducing and nullifying these components you can severely limit algae’s ability to grow.  This is achieved by not overfeeding your koi, by keeping close tabs on water chemistry and making adjustments as needed. 

Make sure your pond isn’t subject to fertilizer runoff as that will often carry a lot of phosphates.  Be sure you have plenty of filtration and beneficial bacteria to assimilate nutrients etc.  You may also need to perform several water changes in an effort to reduce nutrient loads. 

If this is the case be sure that the water changes you do are gradual to ensure that your pond doesn’t undergo a significant pH swing as this may cause harm to your koi.  For more on how biological cycles work see this article.

Adding Salt

Salt seems to be a go-to remedy for a lot of things in the world of koi keeping and it turns out that it can help control algae blooms as well.  There is a caveat with using salt to combat algae in your pond though and that relates to the fact that high enough salinities will also harm or kill your aquatic plants. 

For example, common plants like water hyacinth and lotus will begin to die back at 0.10% whereas water lily won’t die off until 0.5% and to deal with algae effectively you will want to shoot for 0.25 to 0.30%.  You will have to determine if salt makes sense for your algae problems based on your resident species of aquatic plants.  

A good way to determine the right amount of salt is to the use koi-care’s online calculator and once added a simple way to check your salinity is with a commonly used tool called a refractometer.

UV Sterilizers    

One of the most effective ways of combating single celled algae like that which causes “green water” is an in-line UV sterilizer as part of your filtration system.  Its an excellent and non-invasive way of dealing with certain types of algae (and harmful bacteria for that matter) that can easily be added to your existing piping.  So how does UV light kill algae?  UV is a powerful kind of light energy and it effectively penetrates the cell membrane on the outside of the algae cell and hits the areas of the algae where the DNA is (nucleus and chloroplast).  Once this happens, the DNA gets so disrupted by the UV that the algae cell cannot reproduce.

How UV kills algae explained 

UV Clarifiers are also an option  if you are just targeting free floating algae but its less powerful (algae requires less powerful UV to be killed) so if you are going have a UV system you might as well have one that is going to kill other microbes and bacteria, too.  For most ponds a 30 watt system should suffice but be sure that the light you are getting is rated for the number of gallons you have.

Beneficial Bacteria

Besides UV sterilizers one of best things you can do for your pond is adding additional beneficial bacteria.  This is especially true during spring time when your filter media is not ramped up like it would be in summer. 

One of the more popular products on the market for getting your bacteria populations up is called Microbe-Lift PL and they even have seasonal “blends” depending on your needs (and season).  

Its generally a good idea to give your bacteria a boost from time to time but when it comes to algae you may find yourself in a cycle where the algae dies (either naturally or via algaecides), it decomposes on the bottom and causes high levels of nutrients like ammonia and nitrates and those nutrients then fuel the next generation of algae. 

Adding the beneficial bacteria will allow the nutrients to be assimilated before they become available for more algae thereby starving out future algal growth.  

Koi Clay

Koi clay is one of those additions to your koi pond that can only help.   This “stuff” is a natural way to add a lot of great minerals to your system and koi seem to love it.  As a side effect it has been reported to really be effective at inhibiting and killing string algae. 

It is a calcium bentonite clay and when added to your pond it will temporarily cloud it up.  It clears up in a day and will have added lots of beneficial minerals and removed toxins.  It is said that Kentucky produces so many great race horses because they eat the grass growing in Kentucky’s particularly calcium-rich soil.

Similarly, Japan’s koi might be so revered because of the clay rich ponds in which they are raised.  There’s lot of great koi clays on the market but you want ones that don’t remain cloudy for extended periods.


Barley Straw


One algae treatment you may have heard about but is perhaps a bit unexpected is barley straw.  You can get it as raw barley straw or its extract.  This treatment for green water can take up to 30 days to really get going and the results can be hit or miss.  

Some speculate that the barley straw works by breaking down and releasing a toxin that prevents algae while others suggest that the break down process produces hydrogen peroxide which creates a poor environment for algal growth. 

According to Rutgers University no one actually knows how barley straw prevents algal growth but its important to note that it prevents algae, it doesn’t kill existing algae so it shouldn’t be used as an algaecide.  This treatment is more effective on free floating algae as opposed to string algae and is typically used in the spring time. 


Chemicals for Treating Algae


Most algaecides can be placed into one of three categories: potassium permanganate-based, copper-based and simazine-based.  Simazine is a commonly used algaecide.  The way this chemical works is by disrupting the photosynthetic process and thereby killing the algae.  Caution should be used with this chemical as it can harm or stunt the growth of your aquatic plants (as they use photosynthesis, too).   

Potassium Permanganate

Use potassium permanganate with caution.  Not only is it used for parasites like costia but will also readily kill algae however you need to monitor the pond after you add it.  The dose should be around 1 teaspoon per 1000 gallons to start but you may end up adding more or doing more treatments based on your needs and how much algae you have. 

You will need to double up on your pond aeration as you will see a lot of your fish come to the surface and gasp to get air.  Keep up the treatment for about 8 hours and make sure the treated pond water doesn’t go through your filter media as your beneficial bacteria will be killed off. 

Potassium permanganate will get used up as it kills the algae and parasites etc. so you won’t have to do a big water change as you would if you added a lot of salt.  It would be very helpful though to vacuum the bottom of your pond after the treatment is over and your fish aren’t showing signs of stress.

Algae Fix

Algae fix can be placed in the “copper based” category.  Most copper based algaecides are in the form of chelated copper (which lasts longer than other forms).  It can be effective as an algaecide because it disrupts algae cell metabolism however as with most treatments there are some precautions to be aware of. 

Vascular plants like water clover won’t be affected by the copper but other plants that derive nutrients from the water itself may be negatively affected.  The other thing to consider is copper’s affect on invertebrate organisms like snails and crayfish.  Because most invertebrates have copper-based hemolymph (blood) copper-based treatments will harm or kill these organisms.

Green Clean

One of the newer products on the market, “Green Clean“, kills algae via oxidation and results are very rapid.  There is no residue and it is not copper based.  Though it is advertised as a “broad spectrum” algaecide users have reported that it is best for string algae and not ideal for green water (free floating) algae.  The company provides a helpful demonstration video here. All the Green Clean material is in the first 4 minutes.



Accu Clear

This solution is in the family of treatments that cause green water algae to flocculate (suspended materials form small clusters and sink to the bottom).  The idea is that your filter will take care of the rest but as mentioned previously if you don’t vacuum the bottom afterwards it’s a good idea to pump up your beneficial bacteria populations to handle the excess nutrient fallout from the decay of the algae.  


Every pond owner will, at some point, have to deal with algae.  Typically people will struggle with it during the springtime when temperatures rise but fortunately there are plenty of go-to solutions. 

There are some that are broad spectrum and some that are will target one kind of algae or another.  The solution that is right for you is the one that meets your particular needs.  There are plenty of algaecide chemicals available on the market today and a lot of pond owners will attest to their effectiveness.

However, if you find yourself overrun by algae and don’t know where to start try some of the solutions found under “Prevention” in this article first before adding chemicals. You may have to go the chemical route though and if you do be sure to couple those treatments with some of the preventative measures talked about or you may find yourself in the same situation before too long.

Main Points

  1. Shade is a great way to reduce one of the components algae needs to grow: sunlight.
  2. Reducing excess nutrients is another effective way to minimize algae food.
  3. Salt can be effective at keeping algae at bay. Algae likes fresh water so a little salt goes a long way.
  4. Green water algae can’t stand up to UV sterilizers.  The powerful light energy destroys the cells.
  5. Beneficial bacteria are your natural defense against a build up of nutrients that can potentially fuel algal growth.
  6. String algae can be effectively treated with a time-honored application of koi clay.  Koi actually love the stuff too!
  7. Barley straw has been shown to be an effective prevention treatment for future algae episodes.
  8. There’s lot of other ways to attack algae too.  Algae Fix goes after algae cell metabolism, Green Clean is a very effective oxidizer and AccuClear is a flocculator (grabs suspended materials like green water algae).

By |2018-10-08T19:51:21+00:00June 15th, 2014|Algae Problems|72 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. Inday B June 25, 2014 at 6:28 pm - Reply

    Hi, my ex-hubby built a koi pond 9 years ago. Since our separation I got myself busy in my job that I don’t clean the pond regularly. I don’t know anything about the size of our pond, I heard him say it’s a 1600 gal pond with 14 large koi fished. We live in San Diego, CA where it’s always hot. Before water was very clear, but lately it’s getting greener and greener. I try to change the water twice a week but still green. My ex changed the UV bulb since he said it helps kill the algae. It cleared but for a while, now it’s back to green water. Can vacuuming the surface help? Can I call a pond cleaner to vacuum the surface and not shock or kill my koi fishes? Please help, I don’t know what to do anymore.

    • Koi-Care staff June 30, 2014 at 5:52 pm - Reply

      Okay, so first things first -UV sterilizers are great but their bulbs do require changing (just like any bulb) so figure out the manufacturer of the unit and find out where you can get replacement bulbs and what they recommend for a replacement schedule. Also, the surface of the bulb can get dirty -essentially it kills stuff effectively but stuff can still get on and stay on the bulb and cause the bulb to be less effective. Certain UV sterilizer brands actually have a nifty cleaner collar that you swipe down the bulb without ever taking it apart. You really want to get to a point where you are not changing the water so much- you want this thing to be as maintenance free as possible so once your water chemistry is in line (i.e. excess nutrients are kept in check to keep down algae and UV is killing free floating algae) you will be in a better situation. As far as removing much (which everyone has to do) you can get someone to vacuum the bottom of your pond without too much disturbance to your fish but you can also put them in quarantine while that’s being done too -either way they will incur some degree of stress but try to keep it to a minimum. After the vacuuming add a little salt-that will help them get a thicker slime coat on and protect them externally.

  2. Sandie W September 2, 2014 at 3:37 pm - Reply

    I have a small pond with 2 Koi and 4 medium gold fish in it. The pond has a waterfall approx. 3′ that rushes into it. The water is clear, the filter appears to be working fine but there is algae all over each and every rock surrounding my pond. Can I just use a long poled soft brush and brush the algae off the rocks, and, if so will this hurt the fish? Should I brush just a few at a time to lessen the shock to the filter system?

    • Koi-Care staff September 15, 2014 at 3:30 pm - Reply

      Not all algae is bad for your pond and my guess is that the algae in question is short cropped and as you mentioned covers almost every surface. If its not string algae or green water I would be inclined to simply leave it- it produces dissolved oxygen for your fish and they eat it.

  3. Debbie driskill March 21, 2015 at 1:40 pm - Reply

    Have green algae. I can’t get rid of 2200 gallons with koi fish and plants

    • Koi-Care staff April 11, 2015 at 2:51 am - Reply

      Green algae? green water or is it attached? is it long and stringy? the details of the algae will help direct a recommendation for treatment.

  4. THOMAS Bass March 23, 2015 at 9:17 pm - Reply

    Question: If you have a good filter system and your koi pond is 78,000 gallons of water and the filter circulates 505,000 gallons per 24/7 and you drain and clean the bio-filter system and pond that is 25′ x25′ x2’and you have 2 aeration pods and a 1/2 h.p aeration rotary generator a waterfall, you have stream, fountains in a pond that is 80′ x 50′. You have used aqua fix dechlorinators,then three days later add microbes why would the fish be dieing and the pond turns pea green after 4 days there was 56 fish now we have 25. The landscaper has grass surrounding the pond system and I believe the phosphate and nitrates are washing into the pond. we have drained 3 times in 3 weeks. Pressure washed and cleaned the algae off the cement floor and sides. Now, no algae on the cement but in the water. I can send you a photograph of before and after. I’ve used prazi and demilin, while in the holding pond but not at the same time.Everything I have read says we are doing right tasks. What is wrong?

    • Koi-Care staff April 11, 2015 at 2:47 am - Reply

      I would agree with your assessment -I think you are getting nutrients in the runoff from the grass being treated. You really shouldn’t be getting green water without lots of excess nutrients in the system. Unless your filter media is not up to speed with the colony of beneficial bacteria I would think you’ve got a source of nutrients other than that of the fish.

  5. Erin April 28, 2015 at 10:26 pm - Reply

    I have a small pond (450 gallons) that has been overrun with green algae and string algae. I have not had any fish in it for close to a year. I am hoping to clean up and gets more fish. I recently scooped out a good bit of the old leaves and debris and scrubbed the visible string algae off of the rocks.I have done all the chemical tests on the water and they all come back within the normal range. Last week I added some koi clay and some barley straw.the water is starting to clear and looks better. But today when I scooped out some of the leaves I got some strange stringy jellylike substance in the net. It is a crystal-clear gel and organize strings with lines of black seeds inside of it. What is this stuff? It has just shown up in the last week. Could this be from the clay or the barley? I’m not quite sure what to do with it or if I should be worried about it. Please help.

    • Koi-Care staff May 1, 2015 at 2:05 pm - Reply

      sounds like the egg mass of a frog

  6. Garry Berg May 13, 2015 at 4:19 am - Reply

    Where can I buy Gene Winstead’s Ultimate Koi Clay?

    • Koi-Care staff June 3, 2015 at 3:51 pm - Reply

      hmm, that’s strange. Gene seems to have disappeared from the internet. When I added that link to my article it was live and now its a broken link. I can’t seem to find his clay anywhere. He may have gotten out of the business. I supposed you could try Microbe-Lift’s koi clay.

  7. austin tao May 13, 2015 at 6:16 pm - Reply

    I have a 1500 gal water garden w/ gold fish.
    The water is crystal clear and I am very satisfy with that.
    I have round stones at the bottom and along the sides of the pond. They are usually covered w/ algae. How can I get rid of the algae and exposed the natural colors of the stones? I spent several hours physically turning some of the stones over but the algae came back in a matter of 4 days. I don’t mind having some algae covered rocks but not all of them. Can you give me some suggestions as to how I can maintain and get rid of the rock algae?

    • Koi-Care staff June 3, 2015 at 3:50 pm - Reply

      There’s algae that can be harmful to your system and there’s algae that is, more or less, benign. Sounds like what you have is the very short algae that is benign. This algae is simply part of a good ecosystem. Unfortunately I don’t have any suggestions for you other than to pat yourself on the back for having a good, working system. Even the cleanest indoor aquariums will have algae…

  8. Jeri Nicholson June 2, 2015 at 1:03 am - Reply

    I have a large 15,000 gallon pond since 1998. I really enjoyed the article and learned more than I have all the years I have been doing this. Thank you!!!

  9. Kelly June 3, 2015 at 9:06 pm - Reply

    We have just recently moved into a home that has an 800 gallon koi pond with 13 koi. And the one month since we’ve been here the water has turned very green and cloudy with a lot of stringing algae. We are completely inexperienced at dealing with this type of situation. Is there anything you can recommend that we do ?

  10. Cheryle June 9, 2015 at 2:43 am - Reply

    We’re newbies to the koi experience and pond maintenance! We have the type of algae in our 1700 gallon pond as described by Austin Tao. We have clear water and the ph, nitrate, nitrite, etc. levels are all in the appropriate ranges. So happy that this is good, although it is not aesthetically appealing. When periodically brushing off algae, does this stress the (20) koi? Thanks!

    • Koi-Care staff June 20, 2015 at 2:26 am - Reply

      most likely won’t stress them out but as i mentioned its probably being eaten by your koi and if nothing else provides additional dissolved oxygen to your pond.

  11. Pat June 11, 2015 at 5:56 pm - Reply

    Hello, I have a concern about algae in my pond. We have a smaller sized pond, about 110 gallons (that’s a guesstimate) with koi and goldfish. I read through your website some, and we just did our spring clean out a couple weeks ago, we had a pump and added the uv filter, and the water has been beautifully clear. We test the water and it’s good. We do weekly water changes of about 15-20%. We use a shop vac to get debris and the like out of the bottom. I also add beneficial bacteria at least once a week. However the pond liner and rocks are getting green from algae! I don’t want to do another pressure wash, it’s a lot of work and we just did it. What can I do about what’s growing in there?? It’s a bit unsightly..

    • Koi-Care staff June 20, 2015 at 2:24 am - Reply

      Fortunately not all algae is bad and if you have the short (around 6mm or so) kind then you’re doing okay because its just part of pond life. The koi will eat it and it will provide dissolved oxygen for your pond. Otherwise you are going to spend all your time scrubbing liners and rocks and not enjoying your fish! Fish in the natural environment don’t live in algae-free water..

  12. Pat June 20, 2015 at 6:54 am - Reply

    Thank you, I will try to ignore it. It is the short kind of algae, and I do see the fish picking at it. I’m actually quite happy with how much we’ve learned about having an outdoor pond and the fact that it’s thriving.

  13. lloyd June 20, 2015 at 9:36 am - Reply

    what would be the ratio and proportion in putting salt in order to prevent development of algae in a pond

    • Koi-Care staff July 13, 2015 at 3:25 pm - Reply

      0.25% is a good ratio -I am working on a calculator for my site that will allow you to input your gallons and get the amount of salt you need to add to achieve your desired salinity.

  14. Kris Delmage July 6, 2015 at 2:56 pm - Reply

    I have a new 900 gallon pond. It’s about a month and a half old, I’ve been adding the beneficial bacteria weekly and since the warm up (Michigan) I now have green water. The water is clear coming off the waterfall, but cloudy green in the pond. Fish are not visible. I’ve tested the water and all levels are good except ph. Ph runs at 8.2. I’ve tried AquaFix, Barley Extract w/peat moss and AlgaeFix and had no results. Any reccomendations? Super frustrated as the pond meeting is at my house next month. Grrrrr

    • Koi-Care staff July 13, 2015 at 3:16 pm - Reply

      So you could get away with a pH range of 6.8 to 8.2 but the thing wit pH is that you want to avoid big swings in pH as it is logarithmic. As far as the green water algae goes it is probably easier for me to just forward you my algae article -it covers the major algae problems including green water. (http://koi-care.com/getting-rid-of-algae/) By the way, I don’t think barley straw extract will do much -its kind of hit or miss for people.

      • Kris Delmage August 21, 2015 at 11:09 pm - Reply

        Thank you. I’ve since added a uv light and pond was beautiful within a week. But, now I have string algae on the waterfall.

  15. Greg July 10, 2015 at 10:35 am - Reply

    I have pretty clear water in my pond since I added a filter with uv lamp but I have a waterfall with stream leading to pond which we made of white rocks . most of the rocks are covered with brown algae, except a few which are mostly clean I think they might be soapstone or marble. the algae doesn’t seem to grow on them. is there some treatment which will keep the creek bed clean ?

    • Koi-Care staff July 13, 2015 at 3:10 pm - Reply

      Your UV lamp won’t combat algae that is attached to the bottom, only free floating (green water) algae. If the algae you are observing is pretty short (like 1/4 inch) then I wouldn’t worry about it -its just a part of having a koi pond. If you have the longer string algae then it would present a problem.

  16. Robin July 17, 2015 at 2:22 am - Reply

    I have a 3000 gallon pond, actually two ponds with waterfalls and a stream in between. I have never been able to get the Ph down but plants and goldfish are doing great. We have UV which is new this year and the pond is crystal clear. I have just gotten a big string algae outbreak, which plagued me last year but this year has been fine until now. I have lots of lilts so good shade. Any ideas on bringing the Ph down and getting rid of the string algae? Thanks

    • Koi-Care staff July 29, 2015 at 5:45 pm - Reply

      So UV is great for green water algae but string algae is a different beast. I recommend that you read my piece on algae reduction (http://koi-care.com/getting-rid-of-algae/) but koi clay has shown to be effective for string algae. No matter what though you will need to make sure your pond isn’t supporting algae by reducing ammonia and nitrites levels.
      As far as pH goes an acceptable range is 6.8 to 8.2. The way you can bring pH down is by using white vinegar. Just be cautious of big pH swings.

      • Mike August 2, 2015 at 1:51 am - Reply

        When you say white vinegar.. What is the average you would use per gallon?

        • Koi-Care staff August 30, 2015 at 1:38 am - Reply

          around 2-3 ounces for every 500 gallons but it will be a trial and error thing too. Add some, test it after a day or two and go from there.

  17. Patti July 20, 2015 at 12:33 pm - Reply

    Our pond has been cleaned and we have 20 small gold fish. We have a small water fall as well. We have string algae. How do we get rid of that?

  18. Kyle July 30, 2015 at 6:04 pm - Reply

    Is the best way to remove string/weed algae by hand? We have a 1/4 acre pond and at the beginning of the summer the undergrowth ran rampant. Our problem may not solely be algae, but there is enouogh on the surface that it’s unatractive and covers more than half the pond. The stringy, weedy stuff under the surface is another story altogether. We have 1 pump that helps airate the pond, and about 14 ducks. The pond ranges from 6 inches deep to 8 feet deep and is home to many beautiful bass. We’ve been using a treatment more frequently since the algae has become oppressive but nothing seems to be working. It might be time for me to get a net and get slimy. Thanks for any pointers.

    • Koi-Care staff August 30, 2015 at 1:42 am - Reply

      Green Clean” has been shown to be effective against string algae in particular but ultimately algae won’t grow unless it has the right ingredients which include the nitrogenous waste from sources like 14 ducks. So you could certainly get in there and manually remove the algae but chances are good that it will return if the conditions remain right for it to thrive. You may want to reassess your filtration. One of the things that your beneficial bacteria on your filter media do is to assimilate things like ammonia and nitrites that fuel algal growth. So think about a bigger filter or at least adding something like Microbe-Lift PL to boost your beneficial bacteria loads. Also think about more shade for your pond.
      hope this helps -Grant

  19. Kevin August 4, 2015 at 4:29 pm - Reply

    I have a 4000 gal water feature. Main pump is for the the skimmer system, second pump is for the filtration system. The filtration system consists of a 300 gal sediment tank and a 150 gallon filter tank using “bottle brush” filters. The filter system draws off the bottom in two locations. We’ve been battling green water algae all season (NW). Recently did a 70% water change after multiple 10% changes. Prior to the water change, we have been using Algae-Fix and Bacti-Klear and a floculant. No debris in the bottom of the water feature, or creek bed.
    After the water change, the algae was very diluted out but returned quickly, (3 days) even after restarting the Algae fix start up program. PH is 8.0, N02+N03 @ 0, KH @80, GH@30. There are plants in the pond. Also 12 large Koi.
    What am I missing?

    • Koi-Care staff August 30, 2015 at 1:35 am - Reply

      It appears from your water chemistry that you really shouldn’t have this issue. The first thing that comes to mind if you want to really attack the algae is a UV sterilizer however they are not cheap which is why most folks don’t have them. In your case you would need something like a 25 watt unit which would be around $290.00. Next thing that comes to mind is Accu-clear but its also a flocculant like you have already used so it prob. won’t do any good to try another brand. Your nitrites and nitrates are good which means your filtration and beneficial bacteria are doing their job. I guess you could try adding salt but you would want to be cautious of the amount as the existing pond plants will react differently to salt. Sorry i don’t have any better solutions for you.

  20. Taylor September 15, 2015 at 8:37 pm - Reply

    Have a re done 1k gallon pond w/ a 4′ waterfall on one end, skimmer and uv on the other, & bottom drain in the middle (pulls to skimmer). The skimmer has the mat type filter in it, the water fall as well. The pond is mostly sunny and around 4′ deep. Currently the pond has no fish in it as we’ve had a time with animals getting them and fish dieing. Also no plants. We are having the hardest time with stringy algae growing on the rocks and algae growing on the liner (it may be the same type). We were told to use a product called Phosclear for water clarity and Tetra Pond Blocks for the algae. The clarity is fine but the algae is not fazed. The pond is vacuumed 1-2x / month. Any suggestions?

    • Koi-Care staff October 9, 2015 at 3:30 pm - Reply

      first: get plants! not only do they provide shade but they also act as a natural filtration system AND allow fish to hide and not be seen thereby eluding predators.
      second: don’t worry about the algae growing on the liner, my guess is that its the short 1/4 inch cropped kind which is not going to cause any issues.
      third: string algae needs to be taken care of. Tetra Pond blocks I am not familiar with however my customers overwhelmingly prefer Green Clean for string algae (not so much for green water algae, which you don’t have fortunately). In your situation, 1 pound would deal with your string algae and give you enough left over for another full treatment for future use. You can find it here (http://store.koi-care.com/product/green-clean-algae-control/).
      you definitely want to vacuum after treatment to remove the dead string algae b/c what happens is that the decaying biological matter adds to the nutrient loads that go to help future algal growth as well as depleting dissolved oxygen.
      String algae won’t grow to the capacity that you are experiencing it unless it has the fuel for it in the form of sunlight energy and aquatic borne nutrients so the aim as a pond owner is to remove as much of the nutrients as possible and one thing that can help is to boost the beneficial bacteria colonies that you already have. A good way to do that is with Microbe-Lifts PL formula. Just as with the three requirements for fire (heat, oxygen and fuel) if you remove just one of those elements you can’t have fire so analogously if you remove just one of the elements required for string algae to grow you won’t have string alge and beneficial bacteria (http://store.koi-care.com/product/microbelift-pl-3/) and vacuuming will really help that.

  21. Donna December 18, 2015 at 6:14 pm - Reply

    I have a 2000 gal koi pond and it is clear to the bottom, but I just noticed last week string algae starting to form on the sides of the pond(or at least I think that’s what it is, never had before) I’m very confused as to why this would start when the weather is cool. Pond has a cover over it so it doesn’t get much direct sunlight. I put liquid algae treatment in but haven’t seen any change. Was thinking about getting a UV light. Will it would on this type of algae. I’m trying to get rid of it before it gets to bad. Also, only thing that is different this year from last is the water is mostly well water but I was told it wouldn’t hurt the fish.

    • Koi-Care staff December 30, 2015 at 8:27 pm - Reply

      Water doesn’t necessarily have to be warm to initiate algae, just has to have the right chemistry in the water (i.e. fuel in the form of ammonia, nitrites etc. and sunlight). UV is always a great idea but if you are considering one for string algae I wouldn’t get one. UV is great for anything small that floats like viruses, bacteria, and green water algae (it causes that pea soup pond water). String algae is best treated with green clean but there is also koi clay. I always try to get folks to look at the root of the problem too and in this case that would be water chemistry. I would test your water and make sure you don’t have high numbers when it comes to ammonia or nitrites or nitrates. If it were spring/summer I would say to get some microbe lift PL to give your beneficial bacteria a boost. Here is a link to my algae article -I think you will find it helpful. Probably the most popular product I sell is Green Clean, it is especially good at dealing with string algae. There’s even a short demo video on how it works in the article. Ultimately though, the more critical question is: is what you have even string algae? Is it short and fuzzy, maybe 1/2 or shorter? If so, that is algae that I wouldn’t even worry about.

  22. Doug W December 20, 2015 at 4:25 pm - Reply

    We live in Washington State and have a pond that has about 1500 gallons and 5 koi and 3 carp. We started noticing string algae about a week ago. The temperature outside is approx 33 at night to about 47 during the day. We don’t know what to use this time of year. We have never had string algae in the winter. Would love your help. Thank you in advance.

    • Koi-Care staff December 30, 2015 at 8:26 pm - Reply

      String algae is best treated with Green Clean but there is also koi clay. I always try to get folks to look at the root of the problem too and in this case that would be water chemistry. I would test your water and make sure you don’t have high numbers when it comes to ammonia or nitrites or nitrates. If it were spring/summer I would say to get some microbe lift PL to give your beneficial bacteria a boost. Here is a link to my algae article -I think you will find it helpful. Probably the most popular product I sell is Green Clean, it is especially good at dealing with string algae. There’s even a short demo video on how it works in the article. Hope this helps, Grant

  23. brian wolverson January 1, 2016 at 9:17 pm - Reply

    I have a 2000 gallon pond with a bottom drain which leads into strainer then into the filtration system, this then leads to the UV light which then returns back to the pond via a ventura. My ph, ammonia and nitrite levels are all in the acceptable levels. My water has always been crystal clear. About 3 years ago I had trouble with blanket weed which grew to long lengths, I was told to use Clover Leaf this turned my water milky for a couple of days but killed the blanket weed. I have been 2 years free of blanket weed but last year I had a new growth of algea which looks like moss and it has covered all the walls of the pond and has now started growing along the floor. The fish seem to eat it but I’m worried as it’s getting closer to my bottom drain. If it covers it I’m worried my pumps will burn out. I have reused Clover Leaf which turned the moss black, whilst it was black I tried to brush it off the liner but it was still gripping strongly and then it eventually turned green again. I have taken a sample to local koi experts and they have never seen the moss before so couldn’t help, that was about 6 months ago and it’s still green in the plastic bag I took it in. I wonder if there is anything you could suggest that I could use to kill it or alternatively send a sample for someone to look at to confirm what it is.

    • Koi-Care staff March 28, 2016 at 7:44 pm - Reply

      Algae like that won’t typically do really well in a pond unless it has all four components necessary to allow it to thrive (sunlight, dissolved CO2, nutrients and water) -I wonder if you may not have more ammonia or nitrites than you think? Usually, I recommend people use an oxidizer like Green Clean but ultimately your pond has to have sufficient filtration and beneficial bacteria to keep it algae free. The Clover Leaf product you used previously was beneficial bacteria so you could use that again along with an oxidizer like Green Clean. As far as the type of algae I would take some samples to your nearest University -they love to solve mysteries like that!

  24. Dave Eder February 29, 2016 at 4:15 pm - Reply

    I have about a 3,000 gallon koi pond, and right now I have alot of algae that is sticking to all the rocks in the pond, it looks like a small forest, the water is very clear, what can I use to get rid of the algae ? Need Help !

    • Koi-Care staff March 28, 2016 at 7:31 pm - Reply

      how long is the algae? the real short stuff like 1/4 inch won’t negatively affect your pond.

  25. Pam Bernhard March 29, 2016 at 2:17 pm - Reply

    What is the difference between Algaefix and Aquafix? I have the latter product and didn’t see much improvement in my small 500 gal. pond when using it last spring-fall. I have no fish and keep the water dyed as it gets 5-6 hours sun until floating plants cover it. I have trouble with string algae. I keep pond aerated with a pump all winter (live in northern IN) and in spring put filter box and pump in bottom of pond and have water shooting up a ways from pump and also have a waterfall. Do use Spring/Fall clean and Pond Cleaner all from Pondbuilder Co. (from local garden nursery). Thanks.

    • Koi-Care staff July 11, 2016 at 3:45 pm - Reply

      Not familiar with Aquafix but if you are having issues with string algae try koi clay and/or Green Clean. It works fast and won’t harm anything else. There are other more chemical based solutions as well but the two I mentioned don’t need to be approached with caution.

  26. Nik May 8, 2016 at 1:25 pm - Reply


    I had a combination of things happening at the same time.
    1. Weather went from cold to warm in about a week
    2. My UV lamp went off.
    It was in a matter of 4 days that my pond tuned into green soup. Tiny suspended particles

    I have the new UV lamp running but the impact seems none existent. Also tried algo rem from tetra but again no effect.

    I have a 15 000 litre pond with 8 medium koi and 1 shubunkin

    Got a 11K oasis filter and an oxygen pump with 3 air stones attached.

    I am a bit lost as to what to do next? The guy in the local shop is trying to sell me a pond ioniser or more chemicals but I know nothing about ionisers.

    Any suggestions would be more than welcome.

    • Koi-Care staff July 11, 2016 at 3:25 pm - Reply

      Ionizers are actually really great assets to have working for your pond as they deal nicely with free floating algae like you have but they are not cheap -you can read about them here. It isn’t necessary 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.
      Hope this helps.

  27. Jan Rose June 5, 2016 at 1:05 am - Reply

    I would like to know if one cell algae is harmful to my ciclids and lorch and placostimus.
    I have been changing 15 gal of water every 2 days to decrease the algae.
    the algae doesn’t seem to be decreasing; the water is still tourquoise!

    • Koi-Care staff July 11, 2016 at 3:01 pm - Reply

      Sorry for the late reply -I must have missed this one. I don’t believe that free floating algae is a problem in and of itself to your fish however the issue arises when it dies. Upon death the algae sinks, and the bacteria that assimilate the dead algae utilize oxygen in the water and cause low dissolved oxygen levels. Also it increases the amount of free nutrients in the water that allow for the next generation of algae to form as well as other diseases to proliferate. It can also clog your filtration system too.

  28. Cee June 5, 2016 at 1:15 pm - Reply

    We have a drainage ditch that has rocks in it behind are property. We have been here 13 years and this if the first time that it has developed string algae. We have pulled a lot of it out but can’t get it all out What can we do to get rid of this?

    • Koi-Care staff July 11, 2016 at 2:55 pm - Reply

      The root of the problem most likely is due to a recent influx of nutrients. In order to deal with string algae green clean is an effective and fast-acting go to for many folks. You will want to get the dead algae out of the ditch afterwards though or else the cycle will continue.

  29. Paul B. June 22, 2016 at 2:51 pm - Reply

    I have a medium sized koi pond with no fish. Algae is growing up higher on the rocks where the water trickles down from the top. Can I use a slight amount of bleach to get rid of the algae? Thx.

    • Koi-Care staff July 1, 2016 at 7:51 pm - Reply

      Bleach is pretty harsh and not good for any living thing you may have in your pond. Something that a LOT of folks use for algae is Green Clean -its an oxidizer that works on algae immediately but doesn’t leave a chemical residue.

  30. Andy Dyer July 4, 2016 at 4:31 pm - Reply

    I’ve have a 3000 gal. pond, 2-3′ deep, with 19 10′-20′ koi. In South Carolina, the water gets very warm and very green in summer. I’ve got plants (probably not enough), 40 watt UV, skimmer and a spillway, but can’t keep the algae down. Filters are brushes and pads. I would say it’s well aerated, but I’m considering an aeration pump to further support beneficial bacteria to reduce organics and ammonia to try to control the algae. Good idea or no?

    • Koi-Care staff July 11, 2016 at 2:43 pm - Reply

      Yes, always a good idea. One of the most popular things I sell is Microbe-Lift PL. I should ask though, is this algae free- floating like green water or long and weedy? One kind that people worry a lot over is a short, 1/4″ algae that really doesn’t negatively affect your pond or koi (in fact they will eat it regularly). I’m guessing because you have the UV that it is not free floating algae so its either long or short, weedy algae. In that case, when you treat it to get rid of it are you vacuuming out the dead stuff? One thing, that a lot of folks use to get rid of string algae is Green Clean -it works right away and leaves no chemical residue. But as I mentioned, you want to vacuum out the dead stuff or the cycle will begin again.

  31. Patrick Kaye September 13, 2016 at 6:35 pm - Reply

    Should I vacuum the bottom of my koi pond to remove string algae?

    • Koi-Care staff August 8, 2018 at 1:22 pm - Reply

      Its better if you treat the string algae with an algaecide then vacuum.

  32. John Herron December 9, 2016 at 10:41 am - Reply

    I spent 4 years trying to get rid of string algae following many different advices – they all slowed the stuff but it always came back as strong as ever. Then I spotted a small piece about water alkalinity and thought why not – nothing else works. Checked the pH using a good kit and find it at about 9 to 10. Ok – I have a 1200 litre pond with a waterfall and I have 3 litres of hydrochloric acid left from a cleaning job. Let’s try 100ml and see if the fish survive. It didn’t worry the big fish nor the little fish (maybe 100 of them). The pH dropped to 7.4 and I waited. By 7 days the fish were still swimming – the string algae was dying off – the plants took a prolific growth spurt – and I’ve never looked back. It took me two days to broom and filter the pond – and it still looks pristine after 4 months. NO MORE STRING ALGAE!!!!

  33. Johnny Wan January 4, 2017 at 6:32 am - Reply

    Have a water feature with 3 interconnected small ponds (2 pond of 150 litres and 1 pond of 17 lts). It works in a rotation cycle. The two 150 litres pond are at the soil level ,one at the front and one at the back. Water from the pond at the back is pumped upwards to the small intermediary 17 lt pond from which water flows on a 7 step water fall feature down to the front pond of 150 litres , which then goes back to the pond at the back through a pipe.I have installed a Flow through filter set with UV clarifier some two months back which gives me clear water.The front pond of 150 lts and the small pond of 17 lts are covered with slates where as the back pond is hidden under the structure of the water fall feature. I have noticed that the slates along the 7 step water fall as well as the slates on the inside at the bottom and around of the front and upper small pond becomes sticky and has a brownish and greenish colour after 3 weeks. This goes easily away from the surface when brushed and the filtration system effectively does the job by cleaning the water after the slates have been brushed.The 2 ponds have direct sunlight only in the morning and is protected from the sun in the afternoon with tree shades. Though water is clear,the slates do not look aesthetic and clean especially when you look at the ones at the bottom of the two ponds and on the water fall feature. I have not yet put Koi fish up to now and want to be much stabilized on the ponds maintenance before introducing the fish. How can I prevent the slates from becoming sticky and turning into that brownish /greenish colour after only 3 weeks? Is it normal? The shop owner told me that I may put pond clay at the bottom of the two ponds as this will help to prevent the algae from forming too easily on the slates. Please advise. Thank you.

    • Koi-Care staff February 3, 2017 at 8:30 pm - Reply

      Hi, so two things happen a lot in ponds: algae and bacteria. Both of those can be bad and both can be good. It sounds like in your case they are not bad. As you know, in filtration media they are often designed to have lots of crevices and small spaces and that is to promote the growth of beneficial bacteria however those beneficial bacteria don’t just grow on filter media they grow on any submerged surface of your pond, including the slates. Of course, algae will be the same way and certainly there are several types of algae that you really don’t want in your pond but when you have the short (around 6mm) algae that is not a bad thing. Ponds and nature don’t always see aesthetics the same way people do! All things being equal, you could make the slates out of copper and that would inhibit much of the growth…but copper is expensive. In closing, I don’t think the growth is having a negative impact on your pond. Koi clay is never a bad idea. Koi love it and it has been shown to inhibit string algae.
      hope it helps, Grant

  34. Kay Finnie March 29, 2017 at 5:59 am - Reply

    Hello thank you for the great information!!! I feel like a small fry amidst these other owners of giant pond problems!! so our pond is a 150 gallon rubbermaid horse trough. when we returned from a trip end of Feb saw these horse hair stuff floating every where. We removed as much of it as we could and then topped up the water added some new floating plants and now 3 weeks later there is even more scum n heavy floating stuff more than just the horse hair…..please help. We have an external pump with a UV, a small waterfall of 1 feet. There is no run off or debris so not sure why this invasion?? Just started feeding them every 3 days or so from their hibernation. The water is clear if i move the stuff to the side. the fish are prolifically happy from last spring…. may have to cull as 2 are looking ripe 🙂

    • Koi-Care staff August 8, 2018 at 1:06 pm - Reply

      Kay, sorry for the delay. So algae can’t flourish without the proper elements in place: water, carbon dioxide, nutrients and light energy. The usual suspect in these cases is the third one. My guess is that you have an excess of nutrients that is allowing the algae to get a foothold. How is your water chemistry? You have a UV system which is great however when it comes to combating algae its really only effective against the green water algae (the pea soup looking stuff). You can certainly try green clean-its probably my most popular product BUT be sure to extract the dead stuff as much as you can or else the dead stuff will essentially provide the nutrients for the next round of algae. The other thing that I would recommend for this time of year (when everything is starting to ramp up) is a boost to your filter media. The stuff I carry is called Microbe-Lift PL and its basically a ton of beneficial bacteria that then colonize your filter media and actually every nook and cranny IN your pond as well (that’s why its nice to have a lot of nooks and crannies). One caution I would warn against too is when you do water changes be careful not to do huge changes because that has the potential to cause drastic pH swings (which koi really don’t react well to). Hope that helps!

  35. Jackson Bjørn Cooley August 25, 2017 at 8:54 pm - Reply

    Hello, I have a 150 gallon pond for small goldfish etc. I have a small heater in there so the water doesn’t get to cold for the fish as well as a large 300gph pump/filter. I dump about 15ml of Algecide in the water per 3 days yet my pond water always continues to turn a darker and darker green, no matter what I do. I’d like some help with this issue.

    • Koi-Care staff August 8, 2018 at 12:39 pm - Reply

      what does your water chemistry look like?

  36. Dennis Toth August 23, 2018 at 6:11 pm - Reply

    Hi ……..will an inline UV setup kill/eliminate “incoming” (via water changes) string algae? We have two water systems in Grand Junction, Colorado: city water and irrigation water (from the Colorado River). From May thru October we use irrigation water for weekly 10%-15% water changes in our 2400 gallon koi pond. While I’ve been able to control the string algae using Nualgi and Microbe Lift PBL, every irrigation water change seems to “reintroduce” a bloom of string algae. Would a UV light setup for the incoming irrigation water work to eliminate the string algae cells? Thanks.

    • Koi-Care staff September 5, 2018 at 8:40 pm - Reply

      Sorry for the late reply dennis, I must have missed this question. UV is not very effective against string algae as it really targets free floating things like microbes and single cell green water algae.

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