Blue-green algae is cyanobacteria, not true algae. It often forms slimy sheets over substrate, plants and decor and may produce a distinctive earthy or swampy smell. Remove as much as possible manually, siphon trapped waste, improve circulation in dead spots and keep maintenance consistent. Algae-eating fish and snails usually do not fix it. Persistent cases may require a product specifically intended for cyanobacteria, used exactly as directed.
Cyanobacteria can spread frighteningly fast because it forms cohesive mats over surfaces. The most useful clue is its slimy sheet-like growth combined with stagnant areas or a strong earthy smell.
How to recognise it
Blue-green algae can appear green, blue-green, brownish or even reddish. It often peels away in sheets and smells stronger than ordinary algae.
Because it is photosynthetic bacteria rather than true algae, standard algae-eater strategies are often ineffective.
Remove the bulk first
- Siphon loose mats during a water change.
- Gently remove sheets from plants and hardscape.
- Vacuum organic waste trapped beneath the growth.
- Avoid breaking large mats into fragments that spread around the tank.
Fix stagnant conditions
Cyanobacteria often appears where debris settles and flow is weak. Redirect filter outlets or add circulation if a corner consistently collects waste.
Do not create excessive flow for species that prefer calm water; the goal is to remove dead spots, not blast the fish.
Review light and nutrients
Long lighting periods and accumulated organic waste can support outbreaks. Use a consistent photoperiod, keep feeding sensible and maintain the substrate and filter without sterilising them.
When targeted treatment may be justified
Persistent cyanobacteria can be treated with products specifically labelled for aquarium cyanobacteria. Follow the manufacturer's dose and oxygenation instructions carefully.
Avoid casual antibiotic use or improvised dosing because it can affect the aquarium's microbial community and may not be appropriate in every country.
Frequently asked questions
Will algae eaters eat blue-green algae?
Usually not reliably. Cyanobacteria is not true algae and most common algae eaters avoid it.
Why does blue-green algae smell?
Cyanobacteria can produce a distinctive earthy, musty or swampy odour.
Can poor flow cause blue-green algae?
Low-flow, waste-trapping areas are a common place for it to establish, although several factors can contribute.
Sources and further reading
This guide separates general aquarium arithmetic from fish-welfare decisions. For species-specific limits, always check the needs of the fish you intend to keep.