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 a form of aquatic plant life, and there are more than 8,000 species present on earth. Like other plants, it creates energy and growth through the photosynthesis of light. The forms of algae that occur in most fish tanks are freshwater green algae. Most fish tanks have some algae most of the time, 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 tank 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 other plants, algae photosynthesize light. An aquarium in direct sunlight is more prone to an algae bloom.
  • Too many fish, leading to too many organic wastes that provide nutrients for the plants.
  • 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.


  • 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. 
  • Introduce daphnia to the tank. These small crustaceans eat the algae, and they, in turn, are eaten by the fish. 
  • Introduce snails or shrimp to the tank. 

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.


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

As with any aquarium problem, keeping the tank 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.