Green water is a common problem for aquarium owners and pond owners alike. The green color can be unsightly and can distract from viewing the fish. So, can you get rid of it?

While green water may be an eyesore when it occurs in the freshwater aquarium, it can be controlled and eliminated. Understanding how your tank water turns green is the first step towards preventing it.

Algae and the Rise of Green Water

If light and oxygen are present in freshwater, then so is algae. Algae removal will always be a perpetual challenge for the aquarist, and green water algae are no exception. Aquarium algae can grow in nearly any condition the tank may be found. Green water is a form of single-cell, free-floating algae that can rapidly multiply, especially when excesses of oxygen, nutrients, light, or carbon dioxide are present.

While it may be an ugly sight on your tank, green water is common and will not harm fish. It is also a food source for many freshwater invertebrates such as mussels, clams, and other filter-feeders. For those attempting to breed fish, green water makes a nutritious food source for fry that is also simple to culture.

But why is Your Tank Turning Green?

Diagnosing the cause and source of the green color is often difficult for aquarium owners. It can be frustrating trying to control certain problems while not making much headway in the battle against cloudy aquarium water. It should be noted that green water algae can originate from multiple sources, and if the algae have multiple sources of food and energy, it can be challenging to remove.

Sunlight – Probably the single greatest cause of green tank water. The algae thrive on sunlight, especially when high oxygen and fresh water are available. Aquariums placed near windows are high risk of green water algae blooms.

Additional Lighting – If the aquarium has strong or specialized lighting, such as grow lights for planted aquaria, the excess light can also contribute to the presence of algae. Any extra light energy in the environment will contribute to algae growth, including green water.

Overfeeding – Overfeeding fish will result in an excess of organic matter in the water. Not only will this result in a rise in ammonia, nitrites, and nitrates, but it’ll also increase algae. The food won’t just feed the fish, but the aquarium algae as well.

Fertilizers – In planted aquariums where fertilizers and plant foods are used, green water can easily find a home. This is why one must be cautious not to add too much fertilizer to the tank. The green algae love fertilizers, and their use can create significant algae blooms that will cloud water and potentially out-compete plants for nutrients and light if not properly dealt with.

Water Changes – It may seem counter-intuitive, but water changes can actually increase the growth rate of the algae. Like all plants, the algae in utilizes the influx of fresh water, which means more nutrients and trace elements. To compound the problem, the chlorine present in tap water will kill off bacteria in the aquarium water that would naturally feed on green water algae and help control its growth. Thus, water changes actually accelerate the growth of green aquarium water. This doesn’t mean water changes shouldn’t be performed. Make sure to stick to a regular water change schedule, however, do not perform additional water changes in an effort to control or eliminate greenish from your tank.

Remember, green tank water won’t harm fish, though it can be a good indicator that something is wrong with the water chemistry of the aquarium: overfeeding, excess of nutrients, excess light, and so forth. Learn from your algae outbreak and adjust your aquarium practices accordingly.



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