Why Does Aquarium Water Turn Green if There Is No Algae in the Tank?

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Not all algae form mats and film on the surfaces of an aquarium. Even if no algae are visible on aquarium surfaces, free-floating algae can turn the water green. Free-floating algae cause green water. Several remedies exist for the problem.


Green water gets its color from planktonic or free-floating algae. Free-floating algae do not form colonies like the algae that form films or mats on aquarium surfaces. Like some other algae, planktonic algae belong to the protozoans, with larger, more complicated cells than bacteria. Their cells contain chloroplasts like plants, and these algae can produce energy from sunlight.

Effects of Green Water

Green water itself presents little danger to an aquarium, besides making the tank look unclean. However, green water can be a sign of a more serious problem with the lighting or the water chemistry. In extreme cases, algae may block light, preventing plants from getting the light they need to thrive. It is unlikely that green water can pose any direct problems for aquarium fish.

Causes of Green Water

Green water needs two things to thrive: excessive light and phosphate. If a fish tank receives direct sunlight, or the lights are left on for too long, it can encourage the growth of free-living algae. A fish tank in direct sunlight will often develop blooms of green water. Phosphates can come from a number of sources. Fish waste and fish food can be very high in phosphates. In some areas, the tap water is naturally high in phosphates.

Controlling Green Water

You can control green water in several ways. First, limit the amount of light on the fish tank. Never place a fish tank in direct sunlight, since this causes blooms of green water. In many cases, if you leave the aquarium lights off for a few days, the green water will clear up. You can also use a lighting timer, available at most hardware stores, to strictly control how many hours a day your aquarium lights remain on. If phosphates are the problem, consider planting live aquarium plants. Typically, aquarium plants will absorb phosphate more efficiently than algae and cause it to clear up.

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