Green water
Algae bloom

In order to get answers to basic questions all around algae in the aquarium, we recommend you read this article first.

Green water is caused by microscopical green algae that turn the water green. Within a few days they can inhibit the visibility under water significantly. These green algae usually are in the genera Chlorella, Ankistrodesmus and Scenedesmus, however, there are more floating algae from other groups, like e. g. the flagellate protists from the genus Euglena). In natural waters, floating algae are part of the so-called phytoplankton, they are important food for filter-feeding animals like daphnia, mussels and mysid shrimp. Sometimes, green water is the result of a whitish discoloration of the water. These algae float freely in the water and do not settle on surfaces like the glass, decoration or equipment in the tank.

Floating algae, early stage Floating algae, later stage Floating algae, final stage

Common causes

The reasons for a sudden floating algae outbreak can for example lie in seasonal changes like an increase of the temperature during spring and summer or strong sunlight.

In a planted tank or an aquascape, which are fertilized, often nutrient imbalances are the root of an algae problem. Especially in the field of carbon (supplied with CO2) and the macronutrients (NPK), a balanced supply is crucial for the aquatic plants. In the following list you can find the target values for these nutritional elements:

  • A CO2 content of approximately 20-30 mg/l, measurable by a permanent test with test agent
  • 10 to 25 mg/l of nitrate (NO3)
  • 5 to 10 mg/l of potassium (K)
  • 0.1 to 1 mg/l of phosphate (PO4)
  • >10 mg/l of magnesium (Mg)

To prevent unwanted substances from building up, change water regularly. We recommend a weekly water change of 50% of the aquarium volume. Refill the tank with fresh water.

Often, algae blooms occur during the cycling phase of a newly set-up tank. As the young biological system in such a tank is still finding its balance, algae can make use of that situation, and spread considerably. If the new tank has been planted well, and if the aquatic plants in there get sufficient nutrients, it is only a question of time until the algae are outcompeted by the plants.


If you have problems with green water, we can recommend the use of a UVC purification unit. To work as efficiently as possible the unit needs to be connected to a separate pump with a low flow rate. If you connect it to the external filter, the water flow is usually quite high. However, the UV rays work better and kill more of the floating algae in the water if the contact time is as long as possible. After a few days, the algae bloom ought to let up. Change as much water as possible to remove most of the dead algae and to prevent them from polluting the water. Disconnect the UVC purification unit . You should not run it permanently in a planted tank, as it may have a negative effect on the complexing agents used in some fertilisers for aquatic plants.

As an alternative, you could do several large water changes, if necessary in combination with a black-out treatment.

In aquaria with little or no fish you can use algivores that eat floating algae, like daphnia in sufficient numbers.