Causes and Cures for Green Aquarium Water

green aquarium water illustration

The Spruce

That unpleasantly green water that may appear from time to time in your aquarium is caused by a bloom of algae. Algae is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. There are more than 70,000 species present on earth, in both fresh water and sea water, and on land! Like plants, it creates energy and growth through the photosynthesis of light. The forms of algae that occur in most aquariums are freshwater green algae. It is natural for aquariums to have some algae, and in fact, algae can provide food for snails and other aquatic creatures in your collection. The problem comes when the bloom gets out of control and turns the water in your aquarium murky and opaque. The green color occurs because the number of microscopic algae becomes so high that it reduces the transparency of the water. In severe cases, the water may be so green that the fish are not even visible.​


  • Excess light, especially direct sunlight. Like plants, algae photosynthesize light. An aquarium in direct sunlight is more prone to an algae bloom.
  • Too many fish, leading to increased organic wastes that provide nutrients for the algae. Fish wastes act as fertilizer for aquatic plants and algae.
  • Overfeeding. Food that is left uneaten become nutrients used by the algae for growth. 

Green water is usually due to either a significant excess of light (particularly direct sunlight) or a major water quality problem. Although it may look terrible, it is not toxic to fish, unless it reduces the amount of oxygen available in the water (when light is not present).


  • Block out light sources completely. This will shut down the photosynthesis that allows the algae to grow. 
  • Install a diatomic or micron filtration system. This will remove suspended algae in the water. 
  • Install an Ultraviolet (UV) filter. This will kill algae cells that pass through the UV light.
  • Introduce daphnia to the tank. These small crustacea eat the algae, and they, in turn, are eaten by the fish. 
  • Introduce algae eating fish, snails or shrimp to the tank.
  • Add live aquatic plants. These will absorb phosphate, ammonia and nitrate from the water that are also used by algae for growth.

Water changes more frequently will reduce green water temporarily, but will not eliminate it or keep it from coming back, unless the underlying causes are addressed. 


The water aggregators sold in pet stores that profess to clump suspended algae and remove it are not effective against green water.

Also, the use of algaecides that kill the algae may cause a depletion in oxygen in the water due to decomposition of dead algae cells.

Prevention Summary

  • Regular water changes.
  • Regular aquarium cleaning.
  • Use of UV Filter.
  • Avoid direct sunlight on the aquarium.
  • Avoid overfeeding fish.
  • Do not overstock the aquarium.

As with any aquarium problem, keeping the aquarium clean and performing regular water changes is one of the best preventative measures. Prompt attention to sudden algae growth will prevent more serious problems in your tank. 

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  1. Aquarium Fish FAQ. Florida Department of Agriculture and Consumer Services