Green Water (Algae Bloom) in Aquariums

Green water is a bloom of free-floating algae caused by excess nutrients and light. It is cleared with a blackout, a UV sterilizer or a diatom filter.

Identification

Green water is caused by a bloom of free-floating, microscopic algae (phytoplankton) suspended throughout the water column. The water takes on a cloudy green, pea-soup appearance, and discoloration comes from the algae's pigments. It is distinct from surface algae because the organisms float freely rather than attaching to glass or decor.

Causes

An algae bloom is a rapid increase in the algae population driven by an excess of nutrients, particularly nitrogen and phosphorus, combined with light. In aquariums, blooms commonly appear when fish are overfed and the surplus nutrients are not absorbed by plants. Strong or prolonged lighting and ammonia spikes also contribute.

How to clear it

Blackout: cover the tank to exclude all light for about a week, which is generally harmless to fish but stressful to plants.
UV sterilizer: passing water through a UV unit kills suspended algae, often clearing the water within a few days.
Diatom or fine micron filter: physically strains the floating algae out of the water column.
Reduce feeding and perform water changes to lower the nutrient load.

Risk to livestock

Green water itself is generally not directly toxic to fish, but the underlying overfeeding and nutrient excess are unhealthy. Correcting feeding and performing water changes addresses both the bloom and the conditions that caused it.

Prevention

Feed sparingly and remove uneaten food.
Keep the photoperiod moderate and avoid direct sunlight on the tank.
Maintain healthy plants that absorb excess nutrients.
Keep up regular water changes.