Green water

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Green water is a condition, also known as an 'algae bloom', which causes an entire tank's water to turn very green. It can be a blight or a benefit, depending on the situation.


It is a type of a very tiny single-celled algae (Microphytes) which grows suspended in the water of aquarium tanks. This algae can rapidly multiply so that the water turns a dark green in colour which can become almost opaque.

Green water is typically caused by high levels of plant nutrients or leftover fish food, usually accompanied by a highly lit tank.

A bad case of green water can seriously reduce the levels of oxygen in the lower levels of the water and may out-compete the bacteria in the filter for nitrogen, causing a reduction of the size of the bacteria colony in the filter. Therefore if the algae is removed, excessive ammonia may result soon after, as the biological filter can no longer cope. So monitor ammonia levels closely after attempts to remove this condition.


As green water is planktonic algae, there is a train of thought that indicates that the green water is good for fish (if the water is aerated). Goldfish keepers usually are keen to see green water in certain cases.

  1. Algae provide nutrients for newly hatched fry as it can provide an alternative food source for omnivorous and herbivorous fish, especially small fry from egglayers who may sometimes survive only on green water.
  2. It indirectly act as a colour enhancer as they contain natural colour enhancer like chlorophyll fucoxanthin, astaxanthin and other carotenoids.
  3. Infusoria will also develop within the algae on which the fish will feed. These 100% natural (and free) live feeds help develop the intense colouration of your fish and will feed fry and tadpoles.
  4. Good wen growth: It is believed that providing natural foods helps promote the growth of fins and outer skin structures like Lionhead goldfish hoods.
  5. Natural cleanser and oxygenator. The other main benefit of algae is that it helps clean the water in the tank of organic waste and produces oxygen (near the surface or tank walls which are lit). Green algae is, after all, a plant.


Blackout Method

The blackout method is a popular method for treating green water and generally produces excellent] results in most tanks. The blackout method works by denying the algae bloom necessary nutrients it would obtain from fish food and light.


  1. Cover tank completely with thick black trash bags, towels, or other opaque materials. It is important that NO LIGHT be allowed to enter the tank.
  2. Wait 48 hours, during this time do not feed your fish, or peek to see the progress of your tank. During these 48 hours your fish and plants in the aquarium will be fine.
  3. After 48 hours uncover your tank, and you should see crystal clear water. Monitor for ammonia levels afterwards though.

Diatom Filter

It has been suggested by some that diatomaceous earth filters can be used to remove green water. This idea is based on the principle that diatoms' pores are smaller in diameter than the typical algae of an algae bloom so the filter will remove them.

Water Changes

Perform a water change or two to dilute the algae and the nutrients it is feeding off. The owner will also need to reduce light levels and overfeeding otherwise it will come back.


Add more plants to your tank to reduce excess nutrients from the water. This method will take several weeks to work and the plants need light so you also need to do water changes to allow light to penetrate the tank if the green water is bad.


Adding daphnia to the tank if there are no fish in it. They will eat the algae and multiply, so becoming a useful live food source for many species of fish.

See also