Cyanobacteria is a slimy, usually red or purple film that blankets sand and rock in low-flow areas. It isn’t really algae. Clear it by increasing flow where it grows, siphoning it out every few days and checking your nitrate and phosphate. Use a chemical treatment only as a last resort.
How to recognise it
Bulk Reef Supply describes cyanobacteria as a photosynthetic bacterium, often wrongly called red slime algae. It’s usually deep red or purple, but can be green or black. It forms a slimy sheet over sand, rock and sometimes corals, and peels or siphons off easily because it has no roots.
It often traps small bubbles. If the film is brown and dusty and wipes away like dust, it’s more likely diatoms. If it’s stringy, snotty or seems to vanish at night, consider dinoflagellates.
Why it grows
Cyano starts in still water: the sand bed, corners and behind rockwork. Bulk Reef Supply notes that both high and low nitrate and phosphate can contribute, so a test is more useful than a guess. A recent change in feeding, a new light or a reduced water change routine often comes before an outbreak.
How to clear it
Work through these together, over a few weeks.
- More flow where it grows. Point a wavemaker at the patch. Bulk Reef Supply puts this first.
- Siphon it out. Every few days, through a filter sock or during a water change. Blow it off rock with a turkey baster so the filter can catch it.
- Test nitrate and phosphate. If either is high, bring it down slowly; see high nitrate. If both are near zero, see zero nutrients.
- Keep filtration clean. Change filter socks often so siphoned cyano leaves the tank.
- Chemical treatment as a last resort. Bulk Reef Supply says products such as Chemi-Clean can lower oxygen, so add aeration, follow the dose exactly and keep siphoning after treatment.
Stop it coming back
Cyano returns if the dead spots and the cause remain. Keep siphoning for a while after it disappears, and keep the flow changes. Log nitrate and phosphate, and note what you changed and when, so you can see what worked.
Common questions
Is cyanobacteria harmful to corals?
Bulk Reef Supply says it isn’t directly toxic but can smother corals and block their light.
Will a blackout kill cyano?
Some reef keepers use one, but it doesn’t fix the cause. Flow, siphoning and nutrient control are the long-term answer.
Do snails eat cyano?
Most don’t eat it reliably. Siphoning is faster.
Sources and limits
Last checked 12 October 2026. This guide brings together the published advice below. It can’t see your tank: if you’re unsure what you’re looking at, a clear photo to an experienced local shop or club helps.