Blue-green algae (cyanobacteria)
Slimy blue-green or dark sheets that peel off, with a musty smell.
A slimy, blue-green, dark green or reddish sheet over the substrate, plants and decor that peels off in a skin, often with bubbles and a strong musty smell. It is a bacterium, not a true alga.
- Main cause
- Dead spots with little water flow
- First fix
- Siphon off as much as you can and do a large water change; clean the filter.
In a real tank: blue-green algae spreading over the substrate. Photo: Emboli, CC BY 3.0, via Wikimedia Commons
What causes it
- Dead spots with little water flow
- Lots of organic waste: overfeeding, rotting plants, a dirty substrate
- Very low nitrate in some planted tanks
How to remove it today
- Siphon off as much as you can and do a large water change; clean the filter.
- Increase flow into the affected corner.
- A blackout of a few days (lights off, tank covered), often three or four, can weaken it, though results vary and it returns if the cause is not fixed; fish are fine without light for that long. Do a water change after it.
How to stop it coming back
- Feed less, vacuum the substrate and keep flow across the whole tank.
- Check nitrate: in a heavily planted tank with zero nitrate, plants may need fertilizer.