Aquarium Green Water - Causes and how to get rid of it

Aquarium Green Water - Causes and how to get rid of it


One of the plagues for new aquarists is the dreaded green water algae. Green water algae is the accumulation of microscopic photosynthetic organisms called phytoplankton. Phytoplankton are a species of tiny crustaceans. These have chlorophyll cells stored within them allowing them to use photosynthesis as an energy source. Because of this, once they have a foothold in your aquarium, they're extremely persistent and hard to get rid of.

The primary cause of greenwater algae is the ratio of light to available CO2.
Phytoplankton is an animal that requires oxygen to survive and thus doesn't require available carbon like plant species to grow and reproduce. Imagine a seesaw. On one end you have your available carbon, and on the other hand, you have the PAR (photosynthetically active radiation). If you do not know what PAR is, an article can be found here detailing it. When your light intensity goes up, then your available carbon is utilized more by the live plants and that side goes down. When your light intensity goes down, then your plants do not use enough carbon and it goes up. Just like on a seesaw, it is an inverse relationship. The issue with this balance comes when you have excess light. Limited carbon with intense light means you'll end up with a carbon deficiency, if that happens your plants will have a reduced rate of growth which allows green water algae to take hold. This is because they feed on light and excess organics to thrive and reproduce. So if your plants are stunted from excess light, nutrients will build up and the phytoplankton that make up green water algae will take advantage of the opportunity and take over the aquarium.


There are a few commonly held methods for reducing or getting rid of the dreaded green water algae. I will talk about two of them in this article. The first is the easiest method and something you would simply buy and use. That is a UV or ultraviolet filter. This is an external filter which kills the phytoplankton through ultraviolet light exposure. The second method is to address the underlying issue. And that is limiting nutrients and available organic waste to reduce the overall organics in the water. As well as limiting light to the point of blacking out the tank.

The first and easiest way to remove green spot algae in your aquarium is through a UV filter. Make sure you get a filter that is appropriate for the size of your aquarium or it will not be as effective. Let’s explain what happens within a UV filter. Ultraviolet light works by damaging and mutating the bacteria, parasites, and algae that pass through it. Just like excessive sunlight damaging our skin with tans, or mutating into skin cancer with excessive exposure. With a UV filter, if they have a slow rate of flow which means a longer exposure time, and they have enough wattage they mutate or even damage the algae to the point of not being able to reproduce. This is the easiest way to get rid of green water algae but not the way I recommend as it doesn't combat the underlying cause of the abundance of phytoplankton.

The second and final way to combat green spot algae is to restrict nutrients coupled with lowering light levels and blacking out the system. The first step is a water change or multiple water changes. Test for nutrients and keep changing the water until it tests below 20 ppm nitrate which is the maximum recommended level in the long term for aquarium fish. 20-40 ppm should be the maximum acceptable nitrate level for the health of aquarium fish. By reducing the overall nitrate, phosphates, and amount of organic waste you'll cut off one energy method for the phytoplankton. Then the next step is blacking out the tank. Cover the tank with a thick blanket to reduce available light to zero. Ideally during the 5-7 days you blackout the tank you’ll do water changes everyday or every other day. This is because the mass die-off of the phytoplankton can lead to an ammonia spike or pH crash which could adversely affect your fish and bacteria in your filter. But this isn't always necessary. After the 5-7 day blackout is complete, reduce the
light duration or intensity of the light significantly to prevent the recurrence of the carbon-deficient environment which causes green spot algae to go crazy, and that is how you combat green spot algae in an aquarium.


Written by Jacob Thompson

Imagery by AquaticMotiv Llc 

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