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If you have been a pond owner for any length of time, you know the struggles that we constantly face when it comes to string algae. As pond owners, this fast growing nuisance plagues us all. In this article we will discuss what string algae is, where it comes from and how to deal with it. Let’s clean up our water features once and for all.

String algae in pond

Algae comes in two forms, suspended algae and string algae. Suspended algae turns your water a cloudy green color. It’s a small living particulate that is suspended in the water. This type of algae can easily be controlled without chemicals by way of a UV light. Check our article on UV lights for more information about that.

String algae attaches itself to rocks, waterfalls and streams. The fact that it is attached and not free floating means it doesn’t flow through your UV light, thus remaining unaffected. String algae needs to be handled in other ways.

string algae on spill over rocks on a waterfall

If you think you’re off the hook because you have a pondless water feature, think again. String algae effects all water features, not just ponds. We are all susceptible to becoming overrun with this unwanted nuisance. 

What is Algae

Algae is a term that refers to a wide group of photosynthetic eukaryotic organisms according to wikipedia. . This group of organisms has no true roots, stems, or leaves and ranges from being single celled to more complex, what we refer to as seaweed. They all contain chlorophyll, giving it that signature green color. 

So you’ve done your research. You even installed a UV light (link to our UV light article). That  took care of all the free floating algae. But now you are faced with another green menace. This stringy mess that clings to the rocks and hangs down your waterfall. It seems to double every few days no matter how much you remove. 

Is String Algae Bad For A Pond

The short answer is no. String algae is perfectly natural. It is not bad for your pond or the health of your fish. As a matter of fact string algae plays an important role in the health of your pond. When your pond gets over run with string algae, it’s a sign from mother nature that something is out of balance. The string algae is there to correct that. 

The beneficial bacteria in your pond munch on all the organic waste, fish waste, plant matter etc and break it down. When they’re done they leave behind nitrates. The nitrates are then absorbed by the plants in your pond and the cycle continues. The problem occurs when you don’t have enough plants in your pond to absorb all the nitrates.

An overabundance of nitrates can be harmful to fish and other organisms in the pond. That's where the string algae comes in. It is actually mother natures way of correcting the out of balance ecosystem.

What Causes String Algae

You know that old saying “an ounce of prevention is worth a pound of cure”. This holds true when it comes to string algae. If it isn’t taken care of as soon as it is noticed, you could have a real problem on your hands.

String algae is also known as carpet algae. As it grows, it floats on top of the water and begins to take over. This could begin to threaten the life in your pond if left untreated. If it gets too far out of control the top of your pond will be “carpeted” by this fast growing invader.

carpet algae on the surface of a backyard pond

In order for string algae to thrive it needs two things, food and sunlight. Algae gets its energy from the sun through photosynthesis and organic carbon. The organic carbon is in the form of plant matter and fish waste. String algae feeds off these nutrients in your pond. If there is an over abundance of food for the algae it will grow like wildfire. 

String Algae Control - The Basics

There are a few factors to look at when dealing with an algae infestation. Do you have too many fish in your pond? The recommended formula is approximately 300 gallons per fish.

Are you using beneficial bacteria on a regular basis? Do you have enough pond plants in your pond? Covering 40% of the surface area of your pond with plants like water lilies will drastically reduce the amount of sunlight in the water. Each one of these factors play a role in the health of your pond ecosystem.

By reducing or changing these factors, you can bring balance back to the pond and begin to reduce the amount of string algae. By simply eliminating excess nutrients you can starve out the string algae over time.

Steps For Algae Control

The first step to getting your pond back will be, physically remove as much of the string algae as possible. A rake or a pond shark works well for this task. Even if you use a chemical treatment, it is important to remove as much algae as you can first. 

Killing all the string algae quickly will cause a significant reduction of oxygen in the water. This dangerous reduction of oxygen will be harmful to fish and other aquatic life. You will reduce this risk by removing as much of the algae as possible.

removing string algae by hand

Once you’ve physically removed as much of the string algae as you can it’s time for step two. You can handle this one of three ways: 

  1. Use a chemical treatment such as algaefix to chemically kill the algae. 
  2. Use a product like the IonGen 2 (link to our review) the IonGen2 completely eliminates string algae
  3. Use a natural treatment by adding hydrogen peroxide to the water.

Hydrogen peroxide (H2O2) hyper oxygenates the water. This sudden increase in oxygen destroys the string algae. Using 3% Hydrogen peroxide (the kind found at your local drug store) is safe for fish when used in the correct dose. Using 16oz hydrogen peroxide per 1,000 gallons of pond water will eliminate string algae in a matter of days.

Do not add peroxide directly where fish are located. Direct contact with hydrogen peroxide could burn your fishes' gills. If you add it to your skimmer box, by the time it goes through your pump and waterfall, it is diluted enough and safe for your fish.

We used this method and you can see the results in the video below. Our pond cleared up in 7 days. When you give your pond a hydrogen peroxide treatment, you will need to clean your filter media on a daily basis for a week or so, as the dead algae will collect there.

See the results in the video below

Preventative Measures 

Using barley straw, or barley straw extract will prevent the formation of string algae. This is because as the Barley straw breaks down it produces humic acid. Hydrogen peroxide is formed when the humic acid mixes with oxygen in the presence of sunlight.

Barley straw is only used as a preventative measure, it will not kill the string algae that is already in your pond or water feature. It will prevent the formation of new string algae.

Add a few bails to your waterfall unit or in a discreet location around your pond. Replace the bails every couple of months to keep this working effectively.

Reduce the amount you feed your fish. By doing this you will cut down on the amount of waste that is produced. Also cutting down on uneaten food that may add extra nutrients to the water

Use beneficial bacteria on a regular basis in the warm months. This aids in the breakdown of organic matter and aides in water filtration. Once the temperature drops in the cooler months, beneficial bacteria becomes dormant. 

Having a well planted pond is another preventative measure. Plants in and around your pond make it look more natural and appealing. They also gobble up any excess nutrients. This gives the string algae competition when it comes to a food source. A well planted pond restores the balance and reduces string algae.


The battle between pond owner and sting algae is never ending. The key is to slow it down and prevent it before it overruns your pond. Following the above techniques will put you in good shape, preventing it before it becomes an issue. Until next time, enjoy your backyard water garden!



About the author


I love being outdoors and hands on DIY. If you share my passion, you are in the right place. If I can help one person surprise themselves with the success of a project, then this website is a success.

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  1. String algae has such a bad name on most forums I’m glad you’ve mentioned its natures way of looking after the pond… I personally like string algae I honestly don’t care having to go out once a week to remove it manually when ever it gets a big long… I’d much rather do that then use chemicals and have it breakdown adding more more organic waste in my pond… string algae also helps keep your pond water clean as it clings onto dirt. I found this as when I do manually remove the string algae it messes up my clean water 🙂

    1. The look of string algae is a preference. I certainly won’t hurt anything as long as it doesn’t get out of hand. It could potentially choke out the sun from reaching the greater depths of the pond. If you have a serious algae problem it is best to physically remove it if possible. Using algaecide to deal with a large algae problem will have a significant effect on the available oxygen in the pond for other aquatic life.

  2. I was looking forward to getting rid of all my string algae. My pond is 9'x13' at it's widest, 2.5' depth (varies by 6" by rainfall) with a 1.5' rock ledge running most of the circumference, and has a 15' x 2' stream, max depth 2", running to it. I believe we are dealing with perhaps 800 gallons or so.

    Last Thursday i got a 32oz bottle of H2O2 from Target and poured 14 oz into the water uptake that goes back to the top of the stream. By Sunday there was absolutely no visible results, so at the top of the stream where there is a pocket of string algae in one of the spaces for plants – approx 4.5"x8", i dumped the remaining 18 oz in right on top of the pocket of algae.

    Now, two days later, roughly half of that specific pocket of algae has turned that ghostly white, but algae in that same pocket along with all the algae throughout the pond is still alive and well.

    Given the chance that Target had a bad batch of H2O2, i intend to buy another bottle soon and try half of that.

    The three koi have not shown any issues whatsoever.

    Is it possible that my volume calcs are wrong? Though at first i estimated the main pond as 8'x10', the 9'x11' was by tape measure.

    1. I believe your calculations may be off a bit. A 9X13 pond with an average depth of 2.5 is approximately 2,000 gallons. You can use our pond calculator and input any dimensions you wish.

      It is always better to add too little than too much, just for your koi fish’s sake.

      Good luck!

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