Why 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.  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



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 that essentially tint your water a certain color and reduce the available light in your pond.

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.

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.  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 something 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. 

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.

This entry was posted in Pond Care and tagged algecide, barley straw, beneficial bacteria, blanketweed, copper, free floating, green water, horsehair algae, koi clay, koi pond algae, pea soup, potassium permanganate, salt, simazine, string algae, UV sterilizer, water net. Bookmark the permalink.

39 Responses to Why Algae Occurs and How to Get Rid Of It

  1. Inday B says:

    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 says:

      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 says:

    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 says:

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

    • Koi-Care staff says:

      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 says:

    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 says:

      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 says:

    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.

  6. Garry Berg says:

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

    • Koi-Care staff says:

      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 says:

    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 says:

      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 says:

    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 says:

    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 says:

    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 says:

      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 says:

    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 says:

      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 says:

    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 says:

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

    • Koi-Care staff says:

      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 says:

    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 says:

      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 says:

        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 says:

    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 says:

      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 says:

    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 says:

      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.

  17. Patti says:

    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 says:

    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 says:

      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 says:

    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 says:

      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.

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