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How to Remove Green Pond Algae

Updated on June 3, 2014

Ornamental fish and Koi ponds often suffer from algae blooms that turn the pond water green. Ponds that look like they could double for pea soup don’t offer the best view of the star attractions. Fish look washed out (if you can see them at all), even though the water isn’t dangerous to them in any way. The problem is purely aesthetic, but is one that most aquarists find more than annoying.

Why Is the Pond Turning Green?

A pond will turn green if the conditions are right for large scale algae growth. The two most common forms of green algae are string algae and plankton algae -- there are other types, and red and brown algae as well, but none of these will turn your pond water green. Both string and plankton algae thrive in ponds with excess nutrients and an abundant supply of sunlight. The nutrients come from several sources including fish waste, dead and decaying leaves, decomposed fish food, decaying aquatic plants and weeds, as well as chemicals and fertilizers that filter into the pond from the nearby yard. Under these conditions, and in a low pH environment (pH levels below 7.0), algae will proliferate. Use a test kit to check the pH level in the pond and adjust it accordingly by adding chemical buffers. If you have fish in the pond don’t change the pH more than 0.2 units a day or there will be stress put on the fish. Even though most pond fish are hardy, pH changes are one of the surest ways to kill them.

Algae Bloom

In the spring, every pond suffers a major algae bloom. This is simply a part of the life cycle. Beneficial pond bacteria hasn’t recovered from the cold temperatures and usually takes three weeks to a month to catch up and reestablish a balance in the nitrogen cycle. This bloom can be avoided by artificially adding beneficial bacteria to the pond at the first sign of algae. The ammonia converting bacteria will eliminate the food source for the algae thus eliminating the growth.

How Do I Clean the Algae Out?

Cleaning out string algae is a simple matter of using a stick and fishing it out. This can be done economically in a small pond, but in a larger pond it might be better to invest in a few pool skimmers that empty into deep basins. Another option is to pour kosher salt on the offending algae growth and let it sit for a couple of days. This will dry out and kill the algae. Be sure it is kosher salt; iodized salt will not work. Also, no fish may be in the pond during this treatment nor until the water is desalinated. Plankton algae is a bit harder to deal with. The single cells often slip right through the filter media and recirculate in the pond. There are two methods of removing it.

The first is to buy a commercial congealer, often marketed under the term “clarifier.” This will cause the single cells to clump together and fall to the bottom of the pond where the filter can catch them. Once this product is used the filter will have to be cleaned daily until the pond is cleared of algae.

The second method is to remove the nutrients and sunlight so that the algae starve to death. Removing the nutrients might mean feeding your fish less, clearing out all dead plant materials and watching for dead or dying fish. Sunlight is a far easier thing to control if the pond is small enough. A simple pond shade can reduce, or completely eliminate sunlight from reaching the pond, until the bloom is over. The dead algae will be cleared up by the bacteria in the pond as the nitrogen cycle balances.

Maintaining a pond clear of green water is as simple as keeping track of the pH, nutrients and bacterial colonies in it. Adding new bacterial colonies every spring will stop the initial algae bloom and make the rest of the year a matter of simple maintenance instead of becoming a calamity of biblical proportions.


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