The short answer
Blue-green algae is a slimy, blue-green to dark-green film that sheets over substrate, plants and glass, peels off in slippery mats, and gives off a distinctive musty, swampy smell. Despite the name it isn’t really algae at all — it’s cyanobacteria. To clear it: syphon out what you can, run a 3–4 day blackout, improve water flow, and make sure nitrate isn’t bottoming out. It spreads fast, so act early.
Remove it, then black out the tank
Start by syphoning off as much of the slimy film as you can during a water change — it lifts away in sheets. Then run a total blackout: lights off, and cover the tank with towels or a blanket for 3 to 4 days. Cyanobacteria depend on light, and most plants and fish tolerate a short blackout well. After the blackout, do a big water change to remove the die-off.
Fix the conditions that feed it
Blackouts fail if you don’t change what let cyanobacteria take hold in the first place.
- Improve flow. It thrives in stagnant, low-flow areas, so angle your filter output or add a circulation pump to clear dead spots.
- Don’t let nitrate hit zero. Counter-intuitively, cyanobacteria often bloom when nitrate is very low. If tests read near zero, ease off big water changes briefly and let plants and fish supply some.
- Reduce waste build-up. Vacuum mulm from the substrate, don’t overfeed, and keep the photoperiod to 6–8 hours.
Keep it gone
Once it’s cleared, keep flow strong into every corner, maintain a modest photoperiod on a timer, and vacuum the substrate during weekly water changes so waste doesn’t accumulate. Steady, balanced tank conditions are what stop it returning. For the wider algae picture, read our how to get rid of aquarium algae guide.