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How to fight cyanobacteria in the aquarium

Blue- green algae is commonly and unfortunately incorrectly classified as algae. In fact, it belongs to the kingdom of bacteria (Cyanobacteria). Billions of years ago, it had a beneficial effect on the planet, transforming the original, toxic atmosphere into a friendly, livable atmosphere. Currently, however, cyanobacteria cause undesirable and harmful blooms in marine and inland waters. They can release dangerous substances, such as neurotoxins. Their presence in the aquarium is a serious warning signal. You have to react immediately.


The importance of cyanobacteria in the aquarium and the causes of its occurrence


Cyanobacteria are easy to recognize because they cause a gelatinous, blue-green coating. Their consistency is firm but soft. It disintegrates when touched, it also falls off solid elements (e.g. stones). It is often covered with air bubbles. The bacteria emit an unpleasant odor. Cyanosis is a danger to the health and even life of fish. It can also lower the oxygen content in water, which leads to hypoxia in the animals living in it. It is not known whether cyanobacteria inhabiting the aquarium secrete neuro- or hepatotoxins. On the other hand, an attempt to remove it manually is associated with discoloration of the water and the appearance of a stronger odor.

Main causes of blue- green algae occurrence in the tank

  1. Poorly aerated tank, too infrequent use of the filter and pump, or the opposite - overfiltration and too efficient work of nitrifying bacteria.
  2. Water poor in nutrients, no fertilization with a large number of plants. Mainly it's about nitrogen deficiency. The problem is sometimes underestimated, because there are more problems with excess compounds and algae blooms, not bacteria.
  3. There is an imbalance between nitrogen and phosphorus when the first element is too little or similar to the second element. Other macronutrients also have some importance, although they are considered secondary.
  4. Too frequent water changes, adding too much fresh water. Moderation is more than advisable. Typically, water is changed every 1-3 weeks, 20-30%. This is of course an average, the frequency depends on the requirements of the fish, as well as the current situation of water pollution.

How to fight cyanobacteria in the aquarium?

Cyanobacteria are a big problem in the aquarium, so effective control should involve a combination of several methods. In aquarium shops, more specialized preparations for cyanobacteria and diatoms are available. The "cures" last up to 10 days, but they guarantee effectiveness. 

An additional solution is to shade the aquarium for a few days. During this time, aeration should be started, but CO2 fertilization should not be applied. The lack of light during such a period should not harm the plants, but it can eliminate cyanobacteria or support other methods of control. As an alternative, UV lamps are used (in practice they are better for algae). In saltwater aquariums, the efficiency of the skimmer should be improved and the VSV dose reduced. If you have a sand substrate in both types of aquariums, you should introduce organisms that like to dig in it (increasing circulation and aeration). While many algae-eating organisms can be kept in an aquarium, there are no volunteers for cyanobacteria.


For cyanobacteria, as in many other situations, the saying "prevention is better than cure" works best. However, when harmful organisms appear, it is recommended to use several methods of combating them simultaneously. While chemistry is usually used as a last resort, it can be indispensable here.