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 too much fish waste from an overcrowded pond. Old remedies for keeping fish ponds clear include placing floating aquatic plants on the pond, using barley straw and natural algae eaters like water fleas, tadpoles, and snails.
Water lilies and other floating aquatic plants like Water Hyacinth, Fairy Moss, Water Lettuce and Duckweed are excellent for keeping the growth of algae down in your pond. They can easily be spread out over the pond and will compete with the algae for sunlight. Cover two thirds if not half of your pond with floating plants. Since vegetation makes such an impact in controlling algae submerged water plants are also very helpful. Underwater plants like Elodea, Sagittaria, Cambomba and Hornwort are popular plants for reducing algae. Elodea blooms about a month before the others and grows 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 surface of water for more sunlight and subsequent faster growth. Plants can be placed on 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 are recommended for feeding on tufted algae that grows on pond liner or pots. Some snail species are known to eat plants so keep close watch if using this method. One snail per square foot of water surface is recommended for best results. Do not use Great Pond Snails which reproduce quickly and eat aquatic plants. Water fleas are also considered effective as they 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 try placing the sack underneath either of these as the continuously moving water helps keep oxygen around the water and spreads the chemicals the barley creates. Keep the sack(s) in the upper three feet of pond water where they will receive the most sunlight. Warmer water (70 degrees F or over) will see a difference in algae in about two weeks while barley straw will control the algae of colder water in four to six months.