Green Aquarium Water

My aquarium has a greenish or brownish tinge to the water and looks cloudy. The fish look OK but I can hardly see them. It seems as if it is getting worse by the day. What is causing this and what should I do? Please read on for a simplified view of the problem, its causes and a simple course of action to alleviate it….

What is it?

The green in the water is caused by microscopical green algae called phytoplankton. You will notice them when millions of these phytoplankton get together to form a large group. When low in number, these phytoplankton will turn your aquarium a slight hazy-greenish color and as they increase in number so will the unsightly greenish brownish soupy mess thicken in your aquarium.

Will the green water harm my fish?

In a freshwater aquarium environment, many of the fish actually like the soft acidic water that the free-floating algae encourage and thrive in. In my experience even African cichlids that prefer higher PH water seem to be fine in an algae infested aquarium. In a marine aquatic environment free-floating algae is much less common and although not harmful in and of itself it is a sign of water quality degradation, which unaddressed, will harm fish. So, in short, it will not harm your fish but still needs to be addressed as it is unsightly and is a sign that you may have a deeper water quality problem.

What causes it?

  1. Too much light


The reason I put this first is because in my opinion it is the most frequent cause of free-floating algae blooms. The lights should not be overly intense nor on for extended periods. Algae is a plant and plants love and thrive on light. Use a timer and limit the on time to the recommended levels for your particular aquatic environment. Also an aquarium located in a place of direct sunlight 


  1. Stagnant water


Water that is moving does not allow the algae spores to settle, group together and cause an algae breakout. Also, plants like CO2, Oxygen in the water column dissipates CO2. Moving water is oxygenated water, an enemy of algae growth. Many times, all that is necessary to cure an outbreak is the addition of a wavemaker or power head which basically is a motor driven fan that will move the water. Fish like the oxygen rich water that is absorbed into the water column from the fast-moving water surface.


  1. Nutrient imbalance


Over time in our closed aquatic environments various nutrients tend to build up that greatly contribute to water born algae growth. Think phosphates and nitrates.

Phosphates and Nitrate’s tend to build up over time from decomposing foods caused by over feeding or fish waste from an overstocked aquarium.


  1. Unmaintained aquarium


Neglecting timely maintenance is a large part of water born algae blooms. A neglected aquarium will lead to increased organic wastes. Nutrient export is a key for avoiding negative nutrient buildup. When you keep up with regular partial water changes you are obviously removing the bad and renewing the good.

How do you get rid of the green water?

  1. Reduce light


Shut the aquarium lights off until the water clarity is back to normal. With the lights off the water clarity will improve way quicker. I know you love to see your fish under lighting but trust me it will be worth the wait.


  1. Increase water movement


You won’t see this advise in other articles, but trust me it works. Add a wavemaker or even a cheap air stone. Anything that will add more water turbulence. The extra water movement plus the added oxygen that will be absorbed into the water will break up and starve the groups of phytoplankton and get rid of the unsightly green water.  


  1. Ultra Violet Sterilizer


Install an Ultraviolet (UV) filter. This will kill algae cells that pass through the UV light. You don’t need a high wattage UV for this. Depending on the size of your aquarium even a relatively inexpensive In-Tank UV sterilizer as low as 9watts can do the trick.


  1. Chemical Removal


As a last resort use one of the aquarium algaecides on the market and make sure to follow directions and not overdose. I say as a last resort because I hate adding foreign chemicals to my body and I hate adding them to my aquariums. Besides, they usually are only a temporary fix.




Experiencing a free-floating algae bloom is not a cause for panic. The fish are not in immediate danger. But you definitely want to get it remedied and under control. Getting your algae problem under control may be very quick, or it may take some time. When experiencing a free-floating algae bloom, it may take several water changes to get the green discoloration completely out of the water. But only do this once you have the cause of the algae growth under control. Follow the suggestions above and you and your aquarium will be fine and as a side benefit your aquatic environment will be the better for it! 

Are you still having issues after following the above-mentioned advise? feel free to contact us with any questions or experience you had dealing with this issue.