How to Kill Green Water Algae in a Fish Pond

Green water algae are the fastest-growing forms of algae in fish ponds and tanks. They leave a slimy residue on the side of the tank and turn the water in the pond cloudy. Though there are many ways to kill algae in the short term, the problem will return unless you adjust the nitrate balance in the water or introduce algae-eating organisms. Proper filtering and feeding is the key to keeping green water down once you've killed off the algae.

Things You'll Need

  • Algaecide
  • Tarp (optional)
  • Ultraviolet sterilizer (optional)
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Instructions

    • 1

      Choose your algaecide. Nitrate-removers such as Algone work by removing the nitrates that the algae feed on, and are relatively safe for use in fish tanks. Copper-based salts such as copper sulfate kill the algae directly and may harm certain kinds of fish or other pond life. They may also turn your rocks blue. Algaecides such as Pond Care Algae Destroyer also eliminate algae blooms.

    • 2

      Use your algaecide as directed. If using copper salts, the ideal concentration of copper in the water should be 0.15 to 0.20 ppm. In the beginning, the copper readings may be lower than the actual value as algae absorbs the copper initially but releases it when it dies.

    • 3

      Cover your fish pond with a tarp to cut off the light algae need for photosynthesis. Place the tarp on stacked cinder blocks or posts several feet above the pond to avoid cutting off the pond's oxygen supply. This may also kill off plants in the pond but is an otherwise acceptable alternative to chemical additives.

    • 4

      Install an ultraviolet sterilizer to prevent algae growth over the long term. You can buy them at aquarium and pond stores.