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How to control cyanobacteria (Blue-green algae) in an aquarium

March 24, 2020 1 min read

BGA blue green algae cyanobacteria 2HR aquarist

Cyanobacteria, or blue-green algae (BGA) appears as dark green slime. This algae starts as translucent sheets, often along the substrate, and hugging poorly growing plants. It can form filaments that trail in the current. It is often caused by a combination of deteriorating plants, excess organic waste at substrate level, poor oxygen levels, and low nitrates. BGA is actually a photosynthetic bacteria capable of fixing gaseous nitrogen. In large amounts, it smothers plants and plant growth can stall - giving rise to more issues.

Algae eaters often avoid this algae as it can be toxic.

Common Causes

  • Unstable or immature tank bio-filter causing bacteria imbalance
  • Low oxygen or nitrates, coupled with high light
  • Build up of organic waste & detritus
  • Dead spots/low flow areas


  • Small spots can be manually removed or spot dosed with APT Fix.
  • Improving circulation & oxygenation in affected areas can clear mild cases
  • Serious outbreaks may require anti-biotics; Erythromycin. This may impact your nitrifying bacteria though. Commercial products such as UltraLife Blue green slime remover and Chemiclean works as well.
  • Serious outbreaks may be solved by 3-5 day blackout.
  • Water changes and light substrate vacuum to clear organic debris.
  • Increase dosage of nitrates if plants are nitrogen starved. 

Cynobacteria BGA

A patch of cynobacteria / blue-green algae (BGA) on substrate, above.

Cynobacteria BGA

Blue-Green algae appears as a slime across carpet plants above.

BGA blue green algae

BGA on Crypts above.

Here are links to further reading on:

1. Detailed steps on how to cycle a tank

2. Optimizing gaseous exchange and flow patterns