Green unicellular algae will sometimes reproduce so rapidly that the water will turn green. This is commonly called an "algae bloom" and is usually caused by too much light like direct sunlight. An algae bloom can be removed by filtering with micron cartridges or diatom filters. UV sterilizers can prevent the bloom in the first place. Green water is very useful in the raising of daphnia and brine shrimp. Film algae Grows on the aquarium glass and forms a thin haze. Easily removed by wiping the glass. Considered normal with the higher light levels needed for good plant growth.