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title: "Blue-Green Algae (Cyanobacteria): Causes & How to Remove It" description: "The definitive blue-green algae guide: why it's bacteria not algae, the low-flow/low-nitrate triggers, the blackout method, nitrate dosing, and erythromycin as a last resort." slug: blue-green-algae commonName: Blue-Green Algae (Cyanobacteria) scientificName: Cyanobacteria family: Oscillatoriaceae order: Oscillatoriales difficulty: Advanced minTankSize: 1 temperature: "64–86°F (18–30°C)" ph: "6.0–8.5" hardness: "2–25 dGH" lifespan: "Persists and rebounds until conditions change" maxSize: "Sheets covering substrate and plants" origin: "Cosmopolitan — one of Earth's oldest life forms" publishedAt: "2026-06-04"

Blue-Green Algae (Cyanobacteria): Causes & How to Remove It

Blue-green algae is the slimy, foul-smelling sheet that creeps across substrate and plants, peels off in slippery films, and seems to come back overnight no matter how much you scrub. The crucial fact that makes it beatable: it isn't algae at all — it's cyanobacteria, a photosynthetic bacterium, which means it responds to treatments (a blackout, the antibiotic erythromycin) that ordinary algae shrug off. Driven by poor flow, organic buildup, and (counter-intuitively) low nitrate, BGA is one of the few tank nuisances with a fast, reliable cure once you understand it.

This guide is the complete reference: what BGA really is, what causes it, and how to eliminate it with manual removal, the blackout method, nitrate dosing, and erythromycin.

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Species Overview

Blue-green algae (BGA), or cyanobacteria, is not a true algae but a photosynthetic bacterium — one of the oldest life forms on Earth. In the aquarium it appears as a slimy, slippery sheet or film, typically blue-green to dark green (sometimes reddish, black, or brown), that spreads over substrate, plants, glass, and hardscape and peels off in slimy mats. Its hallmark is a distinctive musty, earthy, foul smell — once you've smelled BGA, you'll always recognise it.

BGA's bacterial nature is the key to both its problems and its cure. It spreads fast (binary fission and gliding motility), smothers plants and substrate, and rebounds quickly from manual removal, making it persistent and frustrating. But because it's a bacterium, it's uniquely susceptible to targeted controls — a blackout starves it, and the antibiotic erythromycin eradicates it — that don't work on real algae. It's driven by a specific combination of poor flow, organic buildup, and low nitrate, so the durable fix targets those conditions. BGA is rated "advanced" only because it requires understanding its bacterial nature and root causes; the actual treatments are fast and reliable.

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Natural History and Origin

Cyanobacteria are among the most ancient organisms on Earth — they're the bacteria that, billions of years ago, first produced atmospheric oxygen through photosynthesis. They're found in virtually every environment, from oceans to soil to fresh water, and they're responsible for harmful "algal blooms" in lakes (some produce toxins). In the aquarium, BGA is a freshwater cyanobacterium that forms photosynthetic mats on surfaces.

Its bacterial biology explains its aquarium behaviour. As a photosynthetic bacterium, it needs light and nutrients, but unusually it favours conditions other organisms don't: low-flow, stagnant areas (where it settles undisturbed), high dissolved organics (from detritus, overfeeding, decaying matter), and — counter-intuitively — low nitrate, which gives it a competitive edge because many cyanobacteria can fix their own nitrogen, thriving where nitrate-limited plants and algae can't. This is why BGA often appears in understocked or over-cleaned tanks with low nitrate, poor circulation, and accumulated organics — for example, at the front substrate-glass junction or in stagnant corners. Its ability to glide, spread by fragmentation, and fix nitrogen makes it persistent, but its bacterial nature is its weakness against blackout and antibiotics.

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What Causes BGA

A BGA outbreak is driven by a specific combination of conditions:

• Poor water circulation — stagnant, low-flow areas (the front glass, dead corners, under décor) let BGA settle and spread undisturbed. The most common trigger.
• High dissolved organics — detritus, uneaten food, decaying plant matter, and a dirty substrate feed it.
• Low nitrate — counter-intuitively, very low nitrate favours cyanobacteria (many fix their own nitrogen), giving them an edge over nitrate-limited plants. Common in understocked or over-cleaned tanks.
• Light + nutrients imbalance — like algae it uses light, but the flow/organics/low-nitrate combination is the real driver.

The classic BGA scenario is a tank with stagnant flow, accumulated organics, and low nitrate — which is why it often plagues clean-looking, understocked, or heavily-vacuumed tanks. Understanding these triggers is the key to both treatment and prevention. Confirm your nitrate and tank balance with the nitrogen cycle tracker and water parameters reference.

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How to Get Rid of BGA

Because BGA is a bacterium, it has a fast, reliable multi-step cure — far more so than true algae:

1. Manual removal first:

• Siphon out the slime during a large water change — physically remove as much of the BGA mat as possible (it peels off easily). This is the essential first step before any treatment.

2. Fix the conditions:

• Improve flow/circulation — eliminate stagnant zones with better powerhead/filter placement; this addresses the #1 cause.
• Clean organics — vacuum detritus, reduce feeding, clean the filter.
• Raise nitrate to ~10–20 ppm — dose nitrate (or feed/stock appropriately) to remove BGA's low-nitrate competitive edge. This counter-intuitive step is often the key fix.

3. The blackout method (highly effective):

• After manual removal and a water change, do a complete 3–4 day blackout — cover the tank entirely (no light at all), stop feeding, and keep good aeration/flow. BGA is light-dependent and bacterial, so a total blackout starves and kills it effectively. Most plants tolerate a few days of darkness; do a water change afterward to remove the dead BGA.

4. Erythromycin (last resort):

• For stubborn, severe outbreaks, the antibiotic erythromycin eradicates BGA (it kills the bacteria). Use it carefully: it can disturb the biological filter (the beneficial bacteria), so monitor ammonia/nitrite afterward, dose as directed, and do water changes/carbon after treatment. It's effective but a last resort because of the biofilter risk and general caution around dosing antibiotics.

The reliable approach: siphon it out + improve flow + raise nitrate + blackout, reserving erythromycin for stubborn cases. Because BGA is bacterial, this works far better and faster than fighting true algae.

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Prevention

Preventing BGA is about avoiding the conditions it needs:

• Good circulation — eliminate stagnant, low-flow zones; ensure even flow throughout the tank.
• Maintain some nitrate — don't let nitrate bottom out to zero; keep it around 10–20 ppm so plants outcompete cyanobacteria.
• Control organics — regular water changes, controlled feeding, clean substrate and filter.
• Healthy plant growth — well-growing plants consume nutrients and improve the tank's balance.
• Avoid over-cleaning/understocking to the point of nutrient starvation — a balanced, lightly-nutrient-containing tank resists BGA better than a sterile one.

The recurring theme is that BGA exploits stagnation, organics, and nutrient imbalance (especially low nitrate) — so a well-circulated, balanced, properly-stocked tank simply doesn't get it.

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Is BGA Dangerous?

Some practical health notes: aquarium BGA (cyanobacteria) can produce compounds that, in heavy blooms, may stress fish and deplete oxygen at night (it consumes oxygen in the dark), and some cyanobacteria produce toxins. While a typical aquarium BGA patch isn't usually an acute danger to fish, a heavy outbreak degrades water quality, smothers plants, and can stress livestock, so it's worth addressing promptly. Avoid handling it and then touching your face, and wash your hands after removing it. The musty smell and slimy sheets are also simply unpleasant. The main point: don't ignore a BGA outbreak — its bacterial nature means it spreads fast and degrades the tank, but it's also quickly curable.

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Interesting Facts

• It's bacteria, not algae. BGA is cyanobacteria — a photosynthetic bacterium — which is why it responds to a blackout and the antibiotic erythromycin, treatments that don't work on true algae.
• Among Earth's oldest life. Cyanobacteria first oxygenated the planet's atmosphere billions of years ago.
• It smells. A distinctive musty, earthy, foul odour is a hallmark that distinguishes BGA from other algae.
• Low nitrate favours it. Counter-intuitively, very low nitrate gives nitrogen-fixing cyanobacteria an edge — so raising nitrate helps eliminate it.
• It peels off in slimy sheets. Unlike attached algae, BGA forms slippery mats you can siphon away — the essential first step in removal.

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Bringing It Together

Blue-green algae is, crucially, not algae but cyanobacteria — and that's the key to beating it, because as a bacterium it succumbs to a blackout and to erythromycin, treatments true algae ignore. Driven by poor flow, organic buildup, and low nitrate, BGA appears as slimy, musty-smelling sheets in stagnant, understocked, or over-cleaned tanks. The reliable cure: siphon out the slime, improve circulation, clean organics, raise nitrate to ~10–20 ppm, and do a 3–4 day blackout — reserving the antibiotic erythromycin (used carefully, watching the biofilter) for stubborn cases. Prevent it with good flow, maintained nitrate, controlled organics, and healthy plants. Don't ignore a BGA outbreak — it spreads fast and degrades the tank — but take heart that its bacterial nature makes it one of the more reliably curable nuisances. Keep your tank balanced with the nitrogen cycle tracker, and compare the other stubborn nuisance, black beard algae.