The String Algae Blues

Now that Spring is upon us and things are coming back to life in your pond with the warmer weather, many of us are battling with string algae. String algae is caused primarily by a combination of the buildup of organic nutrients in the water, and sunlight.

What happens is that during winter, when things slow down and many of us turn off our filters, decaying leaves and other organic matter (including fish waste) begin to break down in the pond.

The result is an excess of organic nutrients in the water, which is essentially food for algae growth. Add sunlight and some warmer weather to these conditions, and you have an ideal environment for algae growth, both the green water and string algae varieties.

The green water, or 'pea soup' algae is easily eradicated with a properly sized UV sterilizer, which we highly recommend. String algae, on the other hand, is a bit more complicated and difficult to get rid of.

Because it is usually attached to the walls and rocks of the pond, it becomes more difficult to get rid of and unfortunately, there aren't and easy solutions. But we are going to outline a few remedies here that will help. First, we recommend a pond cleaning - if possible. For those of you who don't clean your pond at least once annually, there are many good reasons to do this.

By cleaning out your pond, and removing as much of the decaying organic material from the bottom, you are also removing much of the 'food' for the algae.

Another good thing to do is to cut back on feeding your fish, until you get your string algae under control. The fish food, both eaten and digested, and the uneaten food, will both also contribute to this algae growth - so don't worry about the fish, just stop feeding for awhile.

There are several products available which can and will 'help' with the string algae problem, like barley bales and/or barley pellets. The idea is that by adding these to the pond, they will also start to decay in the water, and the resulting natural 'humic acids' will actually oxidize the algae, and help keep it under control.

Farmers in the 13th century noticed that lakes with bales of barley straw in them, had much less muck and string algae in them. So, now we use them in fish ponds. Barley pellets are a newer version of the same idea, but will generally start working faster. Barley straw needs several weeks before it starts working, and only after it starts to 'rot' in the water.

Another way to help with string algae is to raise your salt level in the pond. Anything over .3% will help control the algae, but is best for KOI ponds, as the salt may also negatively affect water garden plants.

We also really like the Microbe Lift line of products. They also seem to help control string algae, by adding millions of beneficial microbes and beneficial bacteria that will feed off of this algae and also help to decompose any other organic waste in the pond.

An alternate method that many pond owner prefer is to add a chemical algaecide like PondCare's Algae Fix. This is safe for fish and plants, and does seem to temporarily knock out suspended and filamenous algae, but does need to be added periodically for long term control.

One new method that we've been hearing more and more about is adding a certain type of clay to the pond water. Calcium Montmorillonite Clay is gaining popularity among KOI pond owners and water gardeners as an effective, natural method of algae control. It is also noted as having many beneficial qualities for the fish in the pond.

Hope this helps!

Happy pondkeeping.

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About The Author

Brett Fogle is the owner of MacArthur Water Gardens and several pond-related websites including and He also publishes a free monthly newsletter called PondStuff! with a reader circulation of over 6,000 pond owners. To sign up for the free newsletter and receive a complimentary 'New Pond Owners Guide' for joining, just visit MacArthur Water Gardens>

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