Blue-green algae in the aquarium

Christian Fischer, Cyanobacteria Aggregation2, CC BY-SA 3.0

Contrary to the name, we are not talking about real algae, but about bacteria that have no cell nucleus. Even though they are generally blue, they can often take on a green hue. According to the Leibniz Institute for Baltic Sea Research, this could be the bacteria that belong to the oldest group of organisms.

They can drift upwards through gas bubbles inside the cells. Although many of the bacteria are harmless, caution is required when they occur in large numbers, as they can also be toxic. The formation of blue-green algae can be recognized by cloudy water as well as olive or blue-green streaks and an unpleasant smell.

If a small amount of blue-green algae develops in the aquarium between the substrate and the windscreen at the edge, no measures need to be taken yet.

Causes of blue-green algae in the aquarium

If an aquarium is planted and there is an imbalance in the nutrient supply, blue-green algae can develop. A balanced ratio is given if the following quantities are observed:

  • Magnesium: maximum 10 mg/l
  • Phosphate: 0.1 – 1 mg/l
  • Potassium: 5 – 10 mg/l
  • Nitrate: 10 – 25 mg/l
  • CO2: 20 – 30 mg/l

Regular water changes prevent the individual substances from spreading too much. Once a week the water of half of the aquarium should be changed.

Another cause is microorganismsand other bacteria that move around in the aquarium in unfavourable proportions. If the blue-green algae lack competition, they spread faster themselves. Mostly this is a problem that occurs at the beginning because the ecological balance has yet to be developed. Therefore it is useful to add bacterial preparations . Nite-Out II or Special Blend, for example, are suitable for this.

Measures against blue-green algae

1. If only individual nests of blue-green algae that have not yet grown firmly must be combated, this can be done by suction with a hose.

2. If the infestation is more intensive, it is important to start with a dark cure

At first, all blue-green algae are sucked out. The lighting in the aquarium is switched off and the tank is completely darkened for a few days. For this you can use dark fabrics or cardboard , for example. As the water must be optimally aerated, it is recommended to place the filter outlet higher or to install a bubble stone. During the dark phase, neither CO2 nor liquid fertilization should be added. To speed up the process, algae-eating animals can be added. If the aquarium is littered with many stem plants and ground covers that require light, the darkening period should last a maximum of seven days. If the aquarium is planted with shade plants such as ferns or mosses, the time can be up to 14 days. It is important to check the condition of the plants, animals and algae at least every four days .

To feed the aquarium inhabitants it is possible to switch on the light briefly. After finishing the dark cure a complete water change is advisable. It is normal that stem plants can look a little bare after light deprivation. But this will change again after some time. The method helps, but the cause is not yet eliminated.

3. For a nutrient balance, the potassium supply is increased to 30 mg/l with potassium fertilizer and the nitrate content is adjusted to 20 mg/l. After one week, the water is completely changed and fertilization returns to normal. The resulting plant growth displaces the cyanobacteria.

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