Cyanobacteria (Blue-Green Slime): The Film That Is Not an Alga
Description
Cyanobacteria are not algae: they are bacteria that photosynthesise, which is why they behave differently. They form a slimy film, blue-green, dark green or even reddish, that carpets the substrate, leaves and decoration and lifts off in sheets when touched. They smell of wet earth or mould, and that is the first thing that sets them apart from any alga.
They spread very fast: within a few days they can cover the bottom and smother low plants. Almost no fish or shrimp eat them, so there is no cleaner that will deal with them.
Treatment means removing them by hand and cutting off what feeds them, which is almost always stagnant water and an excess of organic matter.
Why they appear
Dead spots without flow
They grow mostly where the water barely moves: bottom corners, behind decoration and under dense plants. Debris builds up there and the water stagnates.
Treatments
Move the water and vacuum
- Vacuum the cyanobacteria film with a siphon and discard that water.
- Reposition the filter or add a small pump so current reaches the bottom areas.
- Rearrange the decoration so there are no closed-off corners.
Organic overload
Organic debris in the substrate, overfeeding and available phosphate with very little nitrate feed them. New tanks, with an immature filter and plants that have not rooted yet, are the most exposed.
Treatments
Clean up and balance the nutrients
- Change at least a third of the water every week while the outbreak lasts and vacuum the bottom.
- Feed less and remove whatever is left.
- Test the nitrate and do not let it sit at zero while phosphate is present; if it does, adjust the fertilising.
Too much light
A long photoperiod or a light that is too strong for the plants in the tank speeds up their growth: cyanobacteria also feed on light.
Treatments
A few days of darkness
- Vacuum the cyanobacteria and do a large water change.
- Cover the tank with an opaque cloth for three days, with no light at all.
- Remove the cover, do another water change and cut the photoperiod to six or seven hours.
Sources
- GBIFGBIF Secretariat (2026). GBIF Backbone Taxonomy. Checklist dataset. https://doi.org/10.15468/39omei. Consultado vía GBIF.org, 2026-08. Cyanobacteria.
Taxonomy verified as ACCEPTED unless stated otherwise.
Cyanobacteria is a phylum of bacteria, not of algae, and GBIF places it in the kingdom Bacteria. It is catalogued with the algae because that is how people with the problem look for it.
The causes of appearance and the treatments have no citable source: GBIF supports the taxonomy, not the management of algae in aquariums. A source for algae is still to be agreed.