Algae Control in Garden Ponds TT90
Published on May 8, 2014

Algae Control in Garden Ponds - Tech Talk 90

We need to first understand algae before we can understand how to control it.

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 them. At the surface, the algae is unsightly and can clog pumps and filters.

Attached algae are referred to as periphyton. They cover the surfaces of shallow stones and other submerged objects. There are thousands of species of algae, each with its own personality. All are plants, using chlorophyll for photosynthesis. They all need, to varying degrees, water, light and nutrients, and there is usually no shortage of any of them in a fed fishpond.

Which of the thousands of species of algae will become dominant in your pond? Over a five-year period, you may never have the same dominant species of algae twice. A slight change in water chemistry—pH, hardness, nitrogen/phosphorus (N/P) ratio, etc.—can bring about a change in species. The seasons, sunlight, shade, turbidity and many other factors will also allow one species of algae to outcompete the others.

Here are some algae-control methods:

  • Rake—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 be hard to see 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 Phosclear®, when added on a regular basis. Phosclear® is an inexpensive water clarifier. For more information on algae control, see "Alum Lake Treatments" Tech Talk.
  • Algaecides, Commercial—In the right dosages, 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 extract 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 μWs/cm² and flow the entire pond's volume through the unit at least six times per day.