Algae fix guide
Blue-Green Algae
Also called cyanobacteria, BGA, slime algae
A slimy blue-green sheet that peels off in one piece and smells musty.
By Mike Elmira
TL;DR, Blue-Green Algae
Blue-green algae is not actually algae, it is cyanobacteria, and it looks like a slimy dark green or blue-green sheet that peels off in one piece and smells earthy or musty. It usually takes hold where nitrate is low, flow is poor, and detritus has built up, often along the substrate at the front glass. Nothing eats it, so the fix is a physical clean-up, a three-day blackout, more flow, and often a nitrate boost. In a stubborn case an erythromycin-based treatment clears it, but fix the root cause or it returns.
How to spot it
A smooth, slimy film in dark green to blue-green, sometimes with a reddish tinge. It sheets over substrate, glass and plants and lifts off in slippery pieces. The musty smell is the giveaway.
What causes it
- Low or zero nitrate
- Poor circulation and dead spots
- Accumulated detritus in the substrate, often at the front of the tank
How to fix it
- Siphon off as much as you can and do a large water change.
- Do a three-day full blackout: cover the tank completely, with the filter running but the lights and CO2 off.
- Improve flow so nowhere stagnates, and keep the substrate surface clean.
- Dose nitrate back up if it is sitting at zero, cyanobacteria thrives when nitrate bottoms out.
- For a stubborn return, an erythromycin-based product clears it, but only alongside fixing flow and nitrate.
Keep it from coming back
Don't let nitrate hit zero, keep flow strong everywhere, and vacuum detritus before it piles up. Cyanobacteria needs a stagnant, nutrient-starved corner to start.
What eats it
Nothing eats cyanobacteria, and you would not want it to. This one is beaten with a blackout, clean-up and flow, not a clean-up crew.
Frequently asked questions
Direct answers to the questions search engines and AI assistants surface most often about this species.
Is blue-green algae dangerous?
It can be. Cyanobacteria can release toxins and it smothers plants and starves the tank of oxygen if it spreads. It is worth dealing with promptly rather than leaving it.
How do I get rid of blue-green algae?
Siphon it out, do a three-day blackout, boost flow, and bring nitrate back up if it is at zero. That clears most cases. A stubborn one may need an erythromycin treatment, but fix the flow and nitrate or it comes straight back.