Cyanobacteria (red slime)

NuisanceUnsightly and competes for space, but won't kill livestock on its own.

Not an algae — a photosynthetic bacterium that forms a slimy sheet across sand, rock and low-flow corners, usually deep red or purple but sometimes green or black. It smothers what it grows over and it traps gas bubbles as it photosynthesises, which is the classic look.

Make sure that's what it is

It comes off in a sheet, not in strands. If you can siphon a patch away in one piece and it drapes rather than tears, that is cyano. It also regrows over the same spot within days, which is the giveaway that you are treating the symptom rather than the cause.

What it looks like

No photos of Cyanobacteria (red slime) yet. If you have it in your tank, a clear shot here is what the next person will compare theirs against — it's the most useful thing you can add to this page.

Getting rid of it

In the order we'd try them. Every method cites where it comes from, and where the reason a treatment works isn't actually established, it says so.

Flow, export, and the reason it is there

Husbandry

Why it works

Cyano needs light and nutrient, and it colonises the places where detritus settles — which is why it appears in the same dead corners every time rather than uniformly. That makes it a diagnosis as much as a pest: a mat in a low-flow spot is telling you where the tank is not turning over. Raising flow through that area removes the settling substrate, and siphoning the mat exports the nutrient bound up in it instead of letting it dissolve back.

What to do

Siphon the mats out during a water change rather than scrubbing them into the water column. Add or redirect flow into the spots it keeps returning to. Look at the photoperiod, and replace ageing T5 or metal halide bulbs, whose shifting spectrum is a documented contributor.

Careful: Physical removal alone always regrows, usually within days. If it comes back in the same place, the flow answer has not actually been applied there.

Source: What is Red Slime and How to Avoid It (Reef Builders)

Antibiotic and proprietary slime removers

Chemical

Why it works

These clear a bad outbreak quickly and are genuinely useful for buying time. What they are is less clear than the marketing suggests: erythromycin-based dosing kills the cyanobacterial mat, and the widely used proprietary blends state that they contain no erythromycin without disclosing what they do contain. The honest summary is that this suppresses the organism and does nothing to the conditions that grew it, so it is a way to buy a few weeks in which to fix flow and export.

What to do

Follow the product's own protocol exactly — these generally require the skimmer and UV off, carbon removed, and a substantial water change afterwards.

Careful: Cyanobacteria is not the only bacteria an antibiotic reaches. Nitrifying bacteria are hit too, and a large mat dying at once consumes oxygen fast enough to crash a tank — heavy aeration during and after is not optional. This is why this method is listed second. Because the formulation is undisclosed, no product is linked from this entry.

Source: What is Red Slime and How to Avoid It (Reef Builders)