If you have an aquarium, you are probably familiar with the term green water. Green water is simply an algae bloom caused by single cell algae that are green in the color. They are suspended in the water or attach to surfaces in the aquarium. In the presence of both light (sunlight or artificial light) for power and an abundance of dissolved nutrients, their population explodes.

Green aquarium water occurs where this dense population of algae is so thick; it can turn an aquarium water into a thick pea soup appearance. Green aquarium water is extremely hard to eliminate once it has gotten a foothold. Since these are single celled plants, they are often so small that they will pass through the filter and not be trapped.

What is the main causes of green aquarium water?

Over lighting and how to prevent

Green aquarium water often results from allowing in too much light. The only light that should be directed into the tank should be artificial and controlled. It is not a good idea to leave the light running all the time. It should be turned on and off to simulate natural daylight and nighttime periods. Too much time using a good spectrum for plant growth will promote plants, but they should be given rest periods.


If sunlight is allowed to enter the system, it is a powerful engine for the growth of algae. Very good artificial lighting that is left on for extended periods of time may be a cause occasionally. However, in almost all cases, the sun striking the tank sometime during the day is the cause.

The tank should be located in a place where it is not touched by the sun. If the room is extremely bright, it also is best to drop the curtains when the sun is out and brightening the room.

An aquarium background is important in preventing any natural sunlight from striking the aquarium during the day. It should be used if the sun is striking the aquarium from the back especially.
If the tank is situated such that the sun comes in through the side, it is possible to wrap the background around the side as well. This may remove a side from viewing, but the green aquarium water pretty much does that when it is allowed to occur anyway.

Over nutrient and how to control

A second contributing factor is a high concentration of nutrients in the water. In another word, green aquarium water is an indicator that something is out of kilter in the water chemistry. Phosphates and nitrates are often limiting factors for populations of aquatic plants, algae, and other vegetation (source). The over concentration of phosphates and nitrates is an important factor for the explosion of algae.


Phosphate is a primary limiting factor in the growth of fresh water plants and algae. If there is enough of it, the bloom can occur; if there is not enough, the algae can not explode readily. It is impossible to remove all the phosphate from the system. The real goal is to remove enough phosphates in order to prevent the algae to explode, even in the presence of light.

There are a number of ways to try to control phosphates and nitrates:

Aquatic plants also require some phosphates and nitrates as the main nutrient sources to grow as well. Actually rooted plants are better able to compete for limited amounts of phosphates and nitrates. They will often get it before the algae can. This deprivation prevents the algae from getting a momentum going and stops the bloom from occurring.

It is best to combine both of the control of lighting and nutrient concentration to prevent green water. It is worth it. Anyone who has experience it knows how difficult it is to eradicate once it has gained a foothold.

How to quickly clear green water in an aquarium?

Control of the nutrient concentration

Algae control is nutrient control. If you’ve been researching this you have probably come across that phrase once or twice. If you have an algae bloom it can be caused by a number of different factors or a combination of them so you need to find out what the cause is and deal with it. This is what makes green water so frustrating because it could be a single factor or it could be a combination.

There are several things you can try to rid your tank of green water.

  • Make sure that there isn’t too much light coming into the tank.
  • Use a phosphate absorbing material in your filter if its available
  • Reduce the amount of light, less time or intensity or both.
  • Cut down the food and make sure that it’s all getting eaten.
  • Vacuum the gravel to remove all fish waste.
  • Check your fish stock levels. It may be that as fish have grown you have inadvertently overstocked your tank.
  • Use fine filter pads and do more water changes for a while
  • Add fast growing plants which will compete with the algae for nutrients
  • Add CO2 which will encourage plant growth stealing food from the algae
  • Cut back on fertilizer for a week or two
  • Add some floating plants.

Finally be patient. If you find what caused it you can hopefully avoid repeating the same mistake. Sometimes green water will simply clear itself leaving you wondering what happened.

Using additives that gather together micro algae

There is no algaecide I would recommend to kill it, since it can cause stress to the fish and harm to other plant species. The only remedies on the market that might be of use would be various flocculates. These products will cause the algae to clump together so that they can be trapped in the filter.

The clumps are messy and quite gooey. I would suggest not using the standard filter media when a flocculate is used. Replace the media with Poly Wool. This is an extremely cheap media and gets clogged quite rapidly. The mess will need to be removed after less than a day.

Acurel Water ClarifierAcurel Water Clarifier

The best product I would recommend to anyone is the Acurel Water Clarifier that is a flocculent that gathers together the microscopic particles, making them catch in the filter. The Acurel Water Clarifier can clear green water so quickly (under 24 hours) and is safe for your aquarium fish and plants.

This method rarely removes all the cells. The remaining algae cells will immediately begin to multiply once again if the advantageous conditions remain. In order to effectively remove green water, you should control the nutrient concentration and lighting in your aquarium after using this method.

The problem with this type of treatment is that you can’t use it over and over. Overdosing will quite rapidly cause the reverse reaction to occur and the tank will get worse.

Equipment that can remove micro algae

If the problem persists, you might have to find an UV sterilizer or a filter with UV light to kill the algae. The UV sterilizer will destroy the DNA of free-floating microorganisms (including parasites, viruses, algae and bacteria) that are in the water flowing through it. By this way, it effectively kill and prevent these microorganism from multiplication. In the case of green water, the UV sterilizer is the most effective way I know to solve the problem.


Hang-On-Back filter with UV sterilization.

Another effective way to remove the cause of the green aquarium water problem, the algae cells themselves, is with a Diatom filter. The very best is from Vortex Innerspace Products. These units are quite expensive, but they are the best mechanical filters I have ever seen.

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  1. I was on city water never ever had a problem with green water tank was always very clear , I moved now on well water
    filled tank started seeing black growing on glass tank turn green as grass 4 months now not been able to get tank to clear up , I know light don’t cause it cause before we run light on tank 24 hrs a day and tank was Crystal clear , every Spence I put well water in not been able to get it from green and not even running light on tank