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

Cyanobacteria or blue-green algae (BGA) appear as dark green slime. This algae begins as translucent sheets, often along the substrate and hugging poorly growing plants. It can form filaments that drift in the current. It is often caused by a combination of deteriorating plants, excess organic waste at substrate level, low oxygen levels and low nitrates. BGA is actually a photosynthetic bacterium that can fix gaseous nitrogen. In large quantities it smothers plants and plant growth can stall - leading to further problems.

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

Common Causes

• Unstable or immature tank bio-filter causing bacteria imbalance
• Low oxygen levels
• Low nitrogen availability causing plant stress
• Build up of organic waste & detritus
• Dead spots/low flow areas

Solutions

• 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. 

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

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

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