Pond algae control can be one of the most frustrating tasks faced by pond owners. Algae blooms are unattractive and reduce the water’s dissolved oxygen levels at night. They can significantly alter the pH of the water and result in fish kills. However, some algae are beneficial. In controlled amounts, it adds beauty to the pond. Some algae serve as an excellent source of nutrition for fish or other pond creatures. Effectively algae control requires an understanding of the different types of algae and their environmental requirements.

Algae are very primitive plants. Like all plants, they need sunlight and nutrients to grow. In a sunny, nutrient-rich environment, they can grow out of control. Thousands of species of algae exist, but pond owners group them in to three broad categories: Planktonic, Filamentous and Blue-Green.

Planktonic algae float suspended in the water. They are too small to see individually but cause an unmistakable green color when present in large amounts. Planktonic algae are a basic staple in the koi food chain and are beneficial in moderate amounts. However, during the dark cycle of photosynthesis they can use critical amounts of dissolved oxygen and harm fish. These algae are too small to remove with a filter. Chemical control, Biological and UV clarification are effective control techniques.

Filamentous algae appear as long, green threads that attach to rocks and aquatic plants. They usually begin growing in shallow water where there is plenty of sunlight. Large concentrations of nitrate in the water also speed their development. Fish do not eat them and filamentous algae can quickly take over a pond. Mechanical filtration is usually ineffective because the algae do not float freely. When they do break free from their attachment points, they can clog submerged pond pumps. The best control is barley as a preventive measure before it actually occurs. Green Clean works well and on contact. The addition of plants to the pond to compete for the nitrate nutrient works well. Surface plants such as water hyacinth, water lettuce or water lilies also inhibit algae growth by shading the pond. A good rule of thumb is to shade at least 50 percent of a pond’s surface.

Blue-green algae form slimy clumps or mats that float on the surface or lie on the bottom of the pond and are the most difficult pond algae to control. Many species are toxic, and the slimy outer layer prevents effective chemical treatments. The clumps break apart easily, and good water circulation and aeration are the preferred control methods.