Algae is a common problem in backyard ponds. Aside from being unsightly, certain types of algae can clog pumps and water features and reduce the oxygen levels in your pond. As your pond naturally balances and matures over the years, algae may become less of a problem, but an active program of algae control in a pond of any age will ensure the most attractive and healthy environment for your fish and ornamental plants.
Types of Algae
Leave the furry stuff. Not all algae are bad. The short, furry algae that covers your liner is, according to WaterGarden.org, beneficial because it provides oxygen and a food source for fish, snails and other amphibians. It also disguises the liner and provides a more natural appearance.
Remove string and blanket algae manually. Use a broom or skimmer handle to capture the long, surprisingly sturdy strands of string or blanket algae. Inspect it for snails and other pond dwellers, then toss it in the compost heap.
Keep your pond clean. Decaying organic matter like leaves and grass clippings decompose and form sludge. Keep a pool skimmer nearby to remove stray leaves and lawn clippings. If your pond is sited under or near a deciduous tree, consider placing a leaf net over the surface of the pond in the fall to facilitate leaf removal.
Fertilizers and pH
The same fertilizers that make your grass grow have the same effect on algae. If your pond is located in a low spot and subject to rainwater runoff from your yard, redirect rainwater by creating a berm---an elevated mound of dirt---that diverts water away from your pond site.
Elevated pH levels contribute to algae growth. Test for pH regularly, especially if your pond is concrete. Pond supply stores carry a variety of testing kits and water additives that will lower (or raise, if necessary) your pond pH to the recommended levels of 6.9 to 8.5.
Fish and Snails
Algae feeds on fish waste so do not overstock your pond. In addition, too many fish can raise ammonia levels in your pond, creating an unsafe environment for them. According to the University of Illinois Extension, "A general rule for stocking fishponds is to use four to six goldfish and one water lily for every square yard of water surface."
If you want to add more critters to your pond, consider snails. Black Japanese Trapdoor snails will reproduce happily and over winter easily in northern ponds. These snails also eat bottom sludge, another source of algae blooms.
Plants and Shade
Use plants for oxygen, shade and filtering. Algae needs oxygen and sunlight to thrive, and the proper mix of submerged and floating plants combine to reduce both. Floating plants add beauty to your pond, but their light-reducing effects keep water temperatures cooler and prevent sunlight from penetrating the water.
To increase your water's shade, consider a nontoxic pond dye. Available in blue or black, pond dye reduces the amount of sunlight available to algae.
All ponds benefit from the active use of enzymes and biological additives. Products that increase the biological activity of the pond by introducing beneficial bacteria that helps to break down sludge, fish waste and dead algae can improve the overall quality of the water while keeping algae levels under control.