Cyanobacteria aquarium guide: how to get rid of blue-green slime algae

Cyanobacteria (often called "blue-green algae" or "slime algae") is a bacteria, not a true algae. It forms slimy, smelly sheets on sand, plants and decor. Siphon off as much as you can, then improve water flow, clean up waste and use a three to four day blackout; most outbreaks clear without medicine.

It isn't usually an emergency, but it spreads fast, smothers plants and can foul the water as it dies. Wear gloves when handling it, since some kinds can make toxins.

How to identify it

A slimy, smeary coating, usually blue-green or dark green, but sometimes black, brown, purple or red.
Peels off in sheets like a thin skin, often with bubbles trapped underneath.
A strong smell: musty, earthy, or like a swamp. Many people notice the smell before they see it.
Grows on the sand first, especially along the front glass, then spreads over plants and decor.
Grows back quickly, often within a day or two of removing it.

Not cyanobacteria: brown dust that wipes off is diatoms. Tough dark tufts are black beard algae. Green water is a different algae (see algae and cloudy water).

When it's a problem

Normal: a small patch on the sand in a sunny spot, removed and gone.
Early problem: patches that return within days after siphoning.
Serious: sheets covering sand, plants and decor; plants melting underneath.
Emergency: fish gasping at the surface. That usually means oxygen is low or a treatment has crashed the filter. Do a large water change, add air, and test ammonia.

What to do right now

Siphon it out. Use a gravel vacuum or narrow tubing to suck up the sheets. Get as much as you can. Less of it means less to fight.
Do a water change of 25 to 50% with dechlorinated water at the same temperature.
Clean the filter in a bucket of old tank water, not tap water, and make sure nothing is blocked.
Add flow and surface movement. Point the filter outlet across the surface, or add an air stone or small circulation pump.
Cut back feeding for a few days.
Move the tank out of direct sunlight if you can, or block the window.

What to do next

Days 1 to 4: the blackout method. Cyanobacteria needs light. A complete blackout weakens it more than it weakens most plants and fish.

Siphon out as much as possible and do a water change first.
Turn off the lights and cover the whole tank with a blanket, bin bags or cardboard, so no room light gets in.
Keep the filter and an air stone running. Oxygen matters more than ever.
Don't feed, or feed very lightly once, during the blackout.
After three to four days, uncover the tank, siphon away dead slime, and do another water change.

Some plants will look tired afterwards, but most recover. Skip the blackout if your plants are already very weak. Some keepers report that blackouts don't work for them; repeating it once, while fixing the causes below, often helps.

Weeks 1 to 4: fix the cause.

Flow: aim for gentle movement everywhere, with no dead spots. Cyanobacteria grows where water is still.
Waste: vacuum the sand weekly, remove dead leaves, and don't overfeed. See overfeeding.
Light: keep it to 6 to 8 hours a day on a timer. Avoid direct sun.
In planted tanks, check nitrate. Cyanobacteria can take nitrogen from the air dissolved in water, but plants can't. If nitrate drops to zero, plants weaken and cyanobacteria wins. Keeping a small amount of nitrate (low single figures in ppm) helps plants compete.
Old filter media and old bulbs can play a part. Replace bulbs on schedule and keep filters clean.

The erythromycin debate. Erythromycin, an antibiotic, kills cyanobacteria and is sold in some places as a "slime remover". Many experienced keepers use it as a last resort. But there are real downsides:

It can harm the beneficial bacteria in your filter. A laboratory study (Nimenya and others, 1999) found erythromycin slowed ammonia removal in biofilters, so watch for ammonia and nitrite spikes and test daily during and after treatment.
It treats the symptom, not the cause, so cyanobacteria often returns.
Using antibiotics when they aren't needed adds to antibiotic resistance.
Many countries have moved most animal antibiotics to prescription-only (Canada did in 2018; the United States in 2023), so you may not be able to buy it over the counter.

If you and your vet do choose this route, remove activated carbon, follow the label or prescription exactly, finish the full course, and keep a water conditioner and test kit ready. Take extra care with shrimp, snails, scaleless fish, catfish and loaches, and expect some plants to sulk.

Products that help

Gravel vacuum or narrow siphon: the best way to physically remove the slime.
Circulation pump (wavemaker) or air stone: gets rid of dead spots and adds oxygen.
Light timer: keeps light periods short and regular.
Blackout cover: a blanket or dark sheet that fits the tank.
Water test kit: for nitrate, phosphate, ammonia and nitrite.
Phosphate-removing filter media: can help where tap water or food adds a lot of phosphate.
Keep on hand: rubber gloves, dechlorinator, and spare filter sponge.

How to prevent it

Keep good flow across the whole tank, including the sand at the front.
Feed less, and vacuum the sand regularly.
Keep the tank out of direct sunlight.
Replace old bulbs and keep a steady light timer.
In planted tanks, don't let nitrate fall to zero.
Quarantine new plants, which can carry cyanobacteria in.

Why it happened

Cyanobacteria are everywhere and arrive easily on plants, decor or in water. They take off when there's still water, lots of waste, too much light, or plants that are struggling. They're among the oldest living things on Earth and are very good at surviving, so fixing conditions matters more than killing them.

When to get help

Get help if fish are gasping, if fish or shrimp die during an outbreak, or if it returns again and again despite changes. A planted-tank club or an experienced fish store can look at photos of your setup. A vet can advise if you're considering antibiotics.

In the game

Multiple Tank Syndrome doesn't have a cyanobacteria card yet. The related card is "Algae takeover", which only an algae eater can clear. (In real life, very few animals eat cyanobacteria.)

Sources

University of California Museum of Paleontology. Introduction to the Cyanobacteria.
Fish Tank Advisor. Cyanobacteria.
Nimenya H, Delaunois A, and others (1999). Short-term toxicity of various pharmacological agents on the in vitro nitrification process in a simple closed aquatic system. Alternatives to Laboratory Animals 27: 121–135.