Algae Control in Garden Ponds
Planktonic algae are very small, suspended particles that color water various shades of green, brown, olive, and even orange. Pollen from trees and other terrestrial plants is sometimes mistaken for planktonic algae, but pollen usually floats on the surface before sinking. Filamentous algae, commonly referred to as string algae, hair algae, and black algae, typically grow on the bottom and float to the surface when sunlight and photosynthesis provide oxygen bubbles that get trapped in it. At the surface, it is unsightly and can clog pumps and filters. They all need, to varying degrees, water, light, and nutrients, and there is usually no shortage of any of them in a fish pond. Here are some algae-control methods:
Physical removal – Filamentous algae is easily raked out of small ponds.
Light – If you reduce sunlight by shading the pond with trees, shade cloth, arbors, etc., there will be a corresponding reduction in algae. Fifty percent shade is significant, but 70 percent shade should provide fairly clear water. Water can also be shaded with dye (such as Black Vail™), but fish will not be seen very well.
Nutrients – It doesn’t take much. When fish are fed, or fertilizer is added to lilies and other plants, there is no scarcity of nutrients for algae. The biological filter has no effect on the nutrient level, because a biofilter only converts ammonia to nitrate, which is still nitrogen. Water exchange has a limited effect, and is typically not feasible. One nutrient needed by the algae is phosphorus and it can be removed (or tied up) by a water clarifier.
Algaecides, Commercial – In the right dosage, these can selectively kill the algae without harming the other plants or animals. However, their effect may be short-lived, as another crop of algae can “bloom” when the algaecide wears off.
Algaecides, Natural – Things like Barley Straw and Barley Xtract have a longer effect because they remain active in the water.
UV Irradiation – A UV sterilizer will disrupt the reproductive process of planktonic algae and keep your pond clear. Use at least 22,000 µW sec/cm2 and flow the entire pond’s volume through the unit at least six times per day.