Does Lime Kill Slime in Ponds?
Slime or scum floating on the top of a pond begins as a dense mat of filamentous algae growing on the bottom. As the mat produces oxygen, it floats to the surface. Some algae species are bright green and slimy; others look cottony or resemble horsehair floating on the water. There are ways to control pond slime, but adding lime will likely make the problem worse, not better.
Adding dolomitic lime to a pond increases the availability of phosphorus, a key nutrient needed for both aquatic plants and pond algae. It also increases carbon dioxide that aquatic plants and algae need for photosynthesis. If you raise fish that eat plants and algae, adding lime will give them more food, but don't add it to your pond if you want to kill slime.
Conrolling Pond Phosphorus
As phosphorus levels rise in ponds, slime algae increase. Septic tank discharges, storm water runoff and waste from dogs, ducks, geese, cattle and horses all add phosphorus to a pond. To starve algae, reduce the amount of phosphorus entering a pond from those sources. Aluminum sulfate, alum, is the most common chemical used to remove phosphorus already in a pond. When you add alum to water, it forms a "floc," or layer of suspended particles of aluminum hydroxide on the bottom of a pond. Phosphorus in the water settles on the floc and combines with it.
Chemical Algae Controls
Algaecides kill algae; algaestates inhibit their growth and prevent them from blooming. Endothall-amine salt, an algaecide, is most effective in water above 65 degrees Fahrenheit and is toxic to fish, so you have to treat a pond several times a year at low concentrations. Begin treating at the edge of a pond and move outward so fish can escape the chemical. Use a plastic garden sprayer to apply the algecide copper sulfate directly on floating mats of scum or on the water just above submerged mats. For larger ponds, tow burlap bags of copper sulfate crystals behind a boat until all the crystals have dissolved over algae mats. Spraying water-soluble copper called copper chelate on slime will also kill it. Granular forms of sodium carbonate peroxyhydrate, an algaestat, is sold under several brand names.
Non-chemical Slime Solutions
Pond slime usually forms in ponds situated in sunny locations with little shade on the edges. If you have a small pond, add plants to provide shade. Pond dyes are available that absorb sunlight so algae can’t conduct photosynthesis. Lilies, reeds and surface plants will still get the sunlight they need. Black pond dye lasts for several months, although you can use it year-round if you want. A pale blue dye will let you see fish under the water. You can often discourage pond algae by floating a mesh bag of barley straw at the rate of 1 1/2 ounces per square yard in the spring and remove it when it has turned black, about six months later.
- Virginia Department of Game and Inland Fisheries: Pond Management: Renovating Ponds
- Washington State Department of Ecology: Algae Control Methods
- Royal Horticultural Society: Pond Algae and Blanket Weed
- Ohio State University: Controlling Filamentous Algae on Ponds
- Texas A&M University Extension: Planktonic Algae