There are numerous ways to keep your fish pond clear and free of algae, also called pond scum. Green water occurs when large numbers of algae grow on the pond's surface. Algae feeds off sunlight and any excess nutrients in the water, such as excessive fish waste from an overcrowded pond. There are several old-time remedies for keeping fish ponds clear.
Water lilies and other floating aquatic plants like water hyacinth, fairy moss, water lettuce and duckweed easily spread over a pond and will compete with the algae for sunlight. Cover two thirds if not half of your pond with floating plants. Submerged water plants like elodea, sagittaria, cambomba and hornwort are popular plants for reducing algae. Elodea blooms about a month before the others and spreads so quickly it will require thinning by the end of the summer. Ponds dealing with a serious algae problem should have underwater plants moved closer to the water surface for more sunlight and faster growth. Plants can be returned to the bottom of the pond once the problem is resolved.
Tadpoles, snails and water fleas make effective remedies for dealing with green pond water. Be sure to purchase frog tadpoles as opposed to toad tadpoles. Frog tadpoles will reside in water for up to two years before becoming frogs, while toad tadpoles transform into toads fairly quickly. Tadpoles feed on filamentous forms of algae. Ramshorn, trapdoor and apple snails feed on tufted algae that grows on pond liner or pots. Some snail species eat plants, so keep close watch if using this method. One snail per square foot of water surface is recommended. Do not use great pond snails, which reproduce quickly and eat aquatic plants. Water fleas eat floating algae.
Barley straw is perhaps the most effective old remedy for controlling algae. Use one bundle of barley straw for every 1,000 gallons of water in your pond. Barley straw is non-toxic and will not harm fish or other plant life. Purchase barley straw in pouches at garden supply stores or buy bales, depending on the size of the pond. Spread the bales apart and place them in onion sacks or nylon stockings. Tie a string on one end to a large rock and the other end to the barley sack. If you have a fountain or waterfall, place the sack underneath; the continuously moving water keeps oxygen around the water and spreads the chemicals the barley creates. Warmer water (21.1 degrees Celsius or above) will see a difference in algae in about two weeks. Barley straw will control algae in colder water in four to six months.