How to Get Rid of String Algae

This time of year, you might find yourself fighting string algae in your pond. String algae can grow on all surfaces of the pond, including the waterfall. Here are some ways to eliminate string algae.

Remove String Algae by Hand

You can remove the string algae by hand, or using rakes or whatever tools are available to scrape it off the usual growing places like the sides of the pond and waterfall. Once you scrape it off, catch the algae with a net and remove it from your pond. If the string algae are long and growing in the deep end of the pond, you may need a broom handle or pole to wrap the string algae around and pull it to remove it.

Figure Out the Cause of the String Algae

You should try to figure out the cause of the string algae because just removing it and not treating the cause will only mean it will grow back, and the cycle will begin all over again. Usually, string algae grow in high pH level and high phosphorous level environments. Test your water quality to address these issues.

To reduce high phosphorous levels, make sure there are no fertilizers running off your yard into the pond. Also, remove any grass clippings that may have fallen in the pond. It has been proven if you have high phosphorous levels, you can add an Anacharis plant, which thrives on the phosphorous.

Treat Water to Kill Algae

If you feel you need to treat the water with chemicals to kill the algae, I do not recommend using an algaecide as they are harsh for the fish and the environment. You can consider using hydrogen peroxide to treat the water at the application of 3% peroxide, 16 ounces per 1,000 gallons.

I usually prefer using natural ways to treat the string algae, like providing shade cover over the pond area, adding bacteria or natural mineral-based algae additives like barley, increase and enhancing your filtration like adding a trickle tower.

It is important to note that using UV lights can kill other types of algae, but UV lights DO NOT work on string algae.

Add Plants to the Pond

Adding extra plants to the pond will provide extra oxygen and snails that will consume the algae, such as water lettuce or cattails.

Do Not Overfeed Your Koi

When you have a string algae problem, it is important that you do not overfeed the fish. If you are feeding the koi too much food, they won’t do what they naturally know to do, which is graze on the sides and bottom of pond for the algae.

Water Changes

Performing a 20% weekly water change will help reduce string algae.

Test Your Pond Water Parameters

You should regularly, at least once a week, test your pond water parameters for high pH and high phosphate as these are contributors to string algae problems.  If your pH is high and above 8.5, you can consider using a pH stabilizer to bring it down to recommended pH levels of 6.5 – 8.5. Alter the pH gradually no more than 0.2 in a 24-hour period, as too much change could cause a pH crash that is deadly to the koi. 

If your phosphate levels are also high, you will need to treat that with a phosphate remover that is available online or at your local pond store.