Black algae can clog and choke the life out of a fish pond if left unchecked. The algae grows rapidly when exposed to direct sunlight so good preventive measures include the installation of shade plants around the pond so sun does not hit it continuously throughout the day. Algae can also flourish with a sufficient food supply, so it is important to not overfeed the pond fish. Eliminating the problem once it has taken hold of the pond can be accomplished with a suitable algaecide.
Buy an algaecide from an aquarium shop or pet store specifically rated as safe for fish and aquatic plants. Algaecides are sold under several brand names, so it is important to buy a product from a pet store or aquarium shop. Do not use algaecides formulated for outdoor swimming pools.
Estimate the number of gallons your pool contains to determine the correct dosage of algicide. You can calculate the approximate liquid volume by measuring the length, width and depth of the pond in inches, then multiplying them together and multiplying again by 0.00433 (which converts cubic inches into U.S. gallons). For example, the volume of a pond 65 inches long by 50 inches wide and 18 inches deep would be calculated like this: 65x50x18 = 58,500 x 0.00433 = approximately 253 1/3 gallons.
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Add the correct dosage of algaecide to the pond and insert an aerator in the water if the pond does not already have a pump system, fountain, waterfall, or other means of circulating oxygen in the water. Algaecides can cause a dangerous drop in water oxygen levels that can harm fish in ponds without proper air circulation.
Skim dead algae from the pond surface with a pool skimmer or fine-mesh fish net. Most algae will disintegrate and be swept away by the pond pump following treatment with algaecide.