Why Is My Fish Tank's Water Turning Bright Green?

The beginning stages of green water can be seen in this tank.
The beginning stages of green water can be seen in this tank. (Image: Aquarium fish-Capoeta tetrazona image by SPLAV from Fotolia.com)

Green water in aquariums is caused by an explosion in population of a unicellular form of algae called, quite fittingly, green algae. This microscopic type of algae can reproduce extremely quickly, leading to bursts of algae growth that may seem to happen almost overnight. This is especially common in new tanks, but almost any tank at any time can be susceptible to green water if the right conditions are met.

Cycling Issues

New tanks are especially likely to encounter green water problems, and the issue in this case is usually associated with “new tank syndrome,” or cycling. Both of these terms refer to cycle of beneficial bacteria within the tank, which convert ammonia and other harmful chemicals (which are found in decaying fish food and waste) into less harmful byproducts. Bacteria act to compete with algae for these nutrients; because new tanks lack a healthy supply of these bacteria for the first few weeks, green algae is able to thrive unchecked.


The second-most likely reason for green water in tanks (even in older, established systems) is overfeeding. Providing your fish with more food than they can eat in a few minutes leads to large amounts of wasted food, which drift to the bottom of the tank and decompose. Often, this decomposing food provides more waste than the beneficial bacteria can handle, and algae is allowed to bloom. Feeding several smaller meals per day can help remedy this problem.


If an established, cycled tank is encountering green water and overfeeding is not the issue, exposure to sunlight may be the culprit. Make sure the tank does not sit too near a window where direct sunlight could hit the tank at any point throughout the day. Even an hour of direct sunlight per day can potentially cause green water in addition to other types of light-loving algae, such as hair, beard and blue-green algae. Relocating the tank may help solve this.

Tank Lighting

Tank lighting is another common cause of algae growth; this problem can be caused by both lighting that is too high, and lighting that is left on too long throughout the day. Make sure that your light bulbs are rated for aquarium use, and do not use plant-spectrum bulbs unless you have live plants in the tank. Lights should generally not be left on for longer than eight to 10 hours per day. Cutting your photoperiod may help to alleviate the problem.

Poor Maintenance Habits

When a tank’s water is not changed for prolonged periods, nitrate levels in the water are allowed to build, and the tank’s pH can drop. This combination makes it harder for aquatic plants to photosynthesize but has no major impact on the more resilient forms of algae, such as green algae. Regular maintenance is critical to overall tank health, and establishing a maintenance schedule will aid in eliminating future green water outbreaks.

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