What is Green Aquarium Water?


Green aquarium water is a free-floating algae bloom. It can be frustrating to deal with, especially for the beginner fish keeper. Green water, sometimes called ‘pea soup’, can be so severe that the fish literally disappear in it.

The reason for green water

Free-floating algae (photosynthesizing algae) will thrive in nutrient rich waters. Excessive organic waste will eventually pollute the fish tank until it is out of balance. In most cases dissolved organics are abundant and nitrates often test off the charts. Nitrogen/nitrate is the main ingredient in plant fertilizers. Therefore it should come as no surprise that algae will thrive in a nitrate rich aquatic environment.

The need for balance in the fish tank

Some algae growth should always be expected in the fish tank. Once we experience green aquarium water however, we must realize the need to balance the water by controlling the waste and nitrogen/nitrate levels.

While green water and algae blooms also require CO2 and adequate lighting for growth, waste and nitrogen/nitrate are our best means for effective control, as these substances have little to no beneficial use in the fish tank. It is important to understand that green water and algae outbreaks are only the symptoms of the imbalance mentioned above. While treatment of the symptoms, in this case green water, may be possible, a long term solution can only come from prevention through waste and nitrogen / aquarium nitrate control.

In summary:

  • Green aquarium water is a free-floating alga.
  • Excessive dissolved organic waste and high nitrogen/nitrate levels within the fish tank cause green water.
  • The best remedy for green water and algae growth is prevention through waste and nitrogen/nitrate control.

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Patricia Pursell

I am at the point in the last month that I’m having to clean the gravel & change out about 50% of the water like every 3-4 days! I don’t know what’s suddenly going on. I have a 20 gallon tank with 3 goldfish & one small algae eater. I’ve been changing the filter once a month & sometimes sooner. Any advice would be appreciated.


Uhh goldfish are extremely messy fish and a single goldfish requires about 30 gallons! With about 12 gallons for each additional goldfish. They grow to be really big fish. Sorry for the late reply.

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