Bacterial bloom
Milky-white cloudiness in the aquarium water

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

If you have a bacterial bloom in your aquarium, the water becomes cloudy and turns milky within a few days. The clarity of the water is significantly reduced, but no floating particles are visible to the naked eye. This fact enables you to exclude turbidity due to floating detritus and dust. If your water is cloudy due to tiny particles, you can easily remove them with suitably fine filter media. Cloudy water during a bacteria bloom is due to a strong proliferation of free-floating bacteria in the water. Sometimes, the white stain in the water may turn to green. Then an additional algal bloom has developed that was caused by green water.

Bacteria bloom Bacteria bloom removed

Common reasons

Bacteria blooms frequently appear during the cycling phase of an aquarium. The microbiological balance is not established yet as the aquarium still is quite young and the environment inside the tank is more susceptible to these phenomenons. During the cycling phase there might be a more pronounced algal growth, too. This is no reason to fret but quite a normal development.

If all of a sudden, a bacterial bloom occurs in an aquarium that has been running for a longer time, the reason for this may lie in seasonal changes, for example, like temperatures during the hot months or prolonged exposition to daylight.


The only countermeasure that makes really sense is the temporary use of a UVC purification unit. This unit kills any microorganisms in the water that runs through it with the help of UV light. The slower the current inside the unit, the better are the results. Therefore we recommend to connect the UVC purification unit not to your external filter but to a separate pump with a much slower flow. After a few days, the water ought to be crystal clear again. Then we recommend to change the maximum possible amount of water in order to remove most dead bacteria and to reduce the organic pollution in the water that results from them. Now you can put the UVC purification unit away again. Especially in planted aquaria with the corresponding nutrient levels, the prolonged use of a UVC purification unit can have negative effects on the complex builders of some aquatic plant fertilisers, which would have a less than desired effect on the plants in the long run.