Cyanobacteria (Blue-Green Algae) in Aquariums
Cyanobacteria are photosynthetic bacteria, not true algae. They form slimy mats in low-flow, nutrient-rich tanks and are removed by manual cleaning, blackout and water changes.
Identification
Despite the common name blue-green algae, cyanobacteria are not true algae. They are autotrophic, gram-negative bacteria of the phylum Cyanobacteriota that obtain energy through oxygenic photosynthesis. In aquariums they appear as a slimy, slippery blanket coating the substrate, plants and decor, often with a distinctive earthy or musty smell.
Why it is not algae
Cyanobacteria are prokaryotes, meaning their cells lack a true nucleus, whereas algae are eukaryotic. They often live in colonial aggregates and can form phototrophic biofilms. Some species fix atmospheric nitrogen, and a number produce cyanotoxins that can cause harmful effects in animals.
Causes
- Nutrient enrichment of the water (eutrophication), including excess phosphate and nitrogen.
- Calm water with minimal turbulence and low flow, which favours mat formation.
- Poor maintenance, accumulated organic waste and dead spots in the tank.
- Warmer temperatures.
How to remove it
- Manually siphon and remove as much of the slimy mat as possible.
- Perform water changes while gravel-vacuuming to lower the nutrient load.
- Increase water circulation to eliminate stagnant areas.
- A multi-day blackout (covering the tank to exclude all light) can suppress growth; protect plants and livestock during it.
Prevention
- Keep up regular maintenance and avoid letting organic waste accumulate.
- Ensure good water movement throughout the tank, including the substrate surface.
- Avoid overfeeding to limit nutrient build-up.
- Maintain healthy plant growth to compete for nutrients.