How to Kill Brown Algae in an Aquarium

Updated November 21, 2016

Brown algae can be an ugly and harmful problem in your fish tank. Diatom growth can spread over the walls, rocks and decorations of your aquarium and steal nutrients from your aquatic plants. Brown algae commonly grows in newly established tanks, but it can appear in any aquarium that has too many nutrients in the water. Getting rid of brown algae can take time, but by adding a few chemicals, filters and fish, you should be able to get the problem under control.

Chance your tank's water. Use an aquarium siphon to remove about 20 per cent of the tank's water. Make sure to move the siphon through the gravel and substrate to remove fish waste, which can feed the algae. Replace the water using reverse osmosis or deionised water; such techniques use specially filtered water that won't contribute nutrients to algae growth.

Add phosphate adsorption resins to your tank. These chemical filter medias, which can come in pellets or filter pads, will remove phosphates and silicates from your aquarium, helping further starve the algae.

Install an ultraviolet sterilizer outside your tank. This is another water filtration method that will kill brown algae. The UV rays, which are as natural as sunlight, won't harm the fish or plant life but will kill bacteria and algae floating in your tank's water.

Purchase an animal that specialises in eating algae and put it in your aquarium. A few aquatic species thrive on algae, including a freshwater suckermouth fish such as the ancistrus fish or the Siamese algae eater fish. In saltwater tanks, add algae-eating snails such as the margarita snail. All such animals will work like vacuums, eating algae off rocks and tank walls.


Limit how long your aquarium is lit each day. Lighting your tank for more than 10 to 12 hours a day will accelerate brown algae growth.

Things You'll Need

  • Aquarium siphon
  • Reverse osmosis or deionised water
  • Adsorption resins
  • Ultraviolet sterilizer
  • Algae-eating fish or snails
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