How to Kill Blue Algae

Blue algae, also known as blue-green algae and cyanobacteria, is a photosynthetic bacteria that grows in ponds and aquariums. Blue algae is difficult to kill and, unlike true algae, is not eaten by fish. Blue algae will discolor aquarium water, reduce light penetration (which can affect plant life and fish), produce toxins and limit oxygen in an aquarium. It can be cleaned and killed chemically, but this will not destroy the harmful toxins released from the algae, which will continue to deplete oxygen and discolor the aquarium.

Things You'll Need

  • Fishing net
  • Bucket of water
  • Cold running water
  • Clean rag
  • Erythromycin phosphate
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Instructions

    • 1

      Clean out the aquarium. Use the fishing net to remove the fish and place them in a large bucket of water. Remove all plants, gravel and ornaments from the aquarium. Clean the ornaments, plants and gravel in running water and use a clean rag to remove any remaining grit or dirt from the tank contents (including the inside windows of the tank).

    • 2

      Replace the contents of the tank and return the fish. Add erythromycin phosphate to the tank; you will need 200 mg of erythromycin for every 10 gallons of water in the aquarium.

    • 3

      Switch off all lights from the tank and remove all sources of light. The tank should be completely dark for 48 hours, which should kill any remaining blue algae.

    • 4

      Return to the tank and allow light to enter the tank after 48 hours. To control the regrowth of blue algae, you must not overfeed your fish and clean the tank thoroughly and frequently; removing the algae itself without replacing the tank water will eliminate the blue algae but not the toxins that can be potentially dangerous to plant and fish life in the aquarium.