The short answer
Cyanobacteria is the organism behind what most aquarists call blue-green algae — a slimy, blue-green to dark film that coats substrate, plants and glass and smells musty or swampy. The key thing to understand is that it isn’t a true algae: it’s a photosynthetic bacterium. That single fact explains almost everything about how it behaves and how you get rid of it.
Why it’s not really algae
Cyanobacteria are ancient bacteria that photosynthesise like plants, which is why they look plant-like and green. But because they’re bacteria, not algae:
- Algae eaters won’t eat it — shrimp, snails and algae-eating fish leave it alone, so you can’t stock your way out of an outbreak.
- It forms slimy sheets rather than strands or dots, and peels off in slippery mats.
- It has a distinctive smell — a musty, earthy, swampy odour that ordinary algae doesn’t produce.
Why it appears
Cyanobacteria takes hold when tank conditions suit it: low water flow and stagnant dead spots, accumulated waste in the substrate, and often very low nitrate. It’s common in newer tanks, in low-flow corners, and where mulm builds up under-cleaned. A long or bright photoperiod adds to the problem, since it needs light to grow.
How to deal with it
The reliable approach is to syphon out the mats, run a 3–4 day blackout (lights off, tank covered), improve flow into every corner, and keep nitrate from bottoming out. Keep your photoperiod to 6–8 hours on a timer and vacuum the substrate during weekly water changes. For the full step-by-step, see how to get rid of blue-green algae and our how to get rid of aquarium algae guide.