How to Control String Algae

Posted by Cathy Green on

Understanding Algae

There are several types of algae that can befall a pond, but if your water is generally clear with clumps of green floating clinging to your rocks, often forming long strands, then your pond is home to String Algae. String algae can be bothersome for those who enjoy crystal clear water and unencumbered views of their fish and aquatic plants, but it is inevitable! Algae is a natural part of your pond ecosystem that can be maintained just as naturally. To understand how to best control this perturbation, the various causes must be explored.


Algal growth is encouraged by nutrient load, which is affected by fish waste, sun exposure, soil runoff, uneaten fish food, or added tap water. These events contribute to increases in algae fuel: nitrates, nitrates, phosphates, and chloramines that can kill good bacteria. If you’re seeing a sudden increase in growth despite a long history of clear water, you may want to consider these questions:

● Are my fish much larger in size or number than previous years?

● Have any nearby trees or plants been removed to influence the light exposure to the pond?

● Has more tap water been added than usual in response to a leak, evaporation, or repair?

● Has excess fish food been removed after the fish stop eating?

● Has there been extreme weather or rain that may have carried excess nutrients?


First and foremost, we recommend plant and bacterial solutions to naturally reduce algae volume. However, in the winter months, an abundance of plant matter won’t be as readily available. For this season we recommend bacterial solutions such as UltraClear, Algaway, Barley Extract, and Muck Tabs. To determine the most effective solution, the profile of your pond should be considered. If you have fish, UltraClear is a natural treatment that utilizes beneficial bacteria to feed on algae and processes dead debris. Barley Straw concentrate is another great natural option, but is effective over a longer period of use. For more intense treatments or heavier nutrient loads, Algaway is a quick solution that can be used after nutrient-load events. The oxygen-utilizing polyoxyethylene is highly effective in scrubbing away algae, but should be used sparingly to maintain healthy oxygen levels. Fountains and water features are best maintained with Pond Block. The copper sulfate penthahydrate it contains is highly effective against algae, but is safe for fish and birds.


More intense treatments will require high initial doses followed by decreasing doses over a course of weeks. Depending on your nutrient load or filtration, pond clarity can then be retained with a low maintenance dose or natural bacterial additions. Owners who regularly use the beneficial bacterial products UltraClear and Barley Straw in tandem, over a long period of time, report crystal clear water, lower maintenance, and greater resilience to disturbances. For optimal results, we highly recommend the removal of as much string algae before adding treatments to reduce the amount of dead debris.

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