What it looks like

A slimy film of blue-green, less often dark green or black color. It covers the substrate, lower leaves, and glass near the bottom. The main sign is that it peels off in a whole sheet, like a film, rather than rubbing off in pieces. The second sign is a sharp, swampy smell, which can be detected even when the film is just disturbed.

No true algae behaves this way. If the film peels off in a patch and smells, it's them.

Why it appears

These are not algae, but cyanobacteria – blue-green bacteria, and they grow not where there is a lot of light, but where there is:

  • stagnant water: dead spots behind driftwood, areas not reached by the filter's flow;
  • organic matter in the substrate: uneaten food, a layer of silt, dead leaves;
  • low nitrate. As with spot algae, nutrient-poor water makes it worse here: with nitrate near zero, cyanobacteria gain an advantage because some of them can fix nitrogen directly from the air, while higher plants cannot.

What to do

Mechanical cleaning is mandatory, but it alone will not be enough: the film grows back in a few days.

  1. Siphon off all visible parts along with the top layer of substrate where it resides.
  2. Eliminate stagnation: reorient the filter outlet, add flow to dead spots.
  3. Raise nitrate to 10–20 mg/L if it is near zero.
  4. Blackout for three days. The aquarium is completely covered from light – with a blanket, cardboard – the filter and aeration are left on, feeding is stopped. Cyanobacteria tolerate darkness worse than plants and die almost completely within three days. Adult fish are not harmed by three days without light and food; plants endure this much more easily than being completely overgrown. Fry and very small fish cannot be starved this way – they are fed briefly, under dim flashlight, and immediately covered back up.
  5. After the blackout – a large water change and removal of dead matter.

If the film returns after two weeks, it means the cause remains: almost always it is stagnation or organic matter in the substrate.

What not to do

Do not delay. By growing in a sheet, cyanobacteria block light and water access to the leaves, and the plants underneath them rot in a matter of days. This is a case where waiting for it to "go away on its own" is not advisable: it won't go away on its own.

Do not rely on fish. Nothing eats them.