Knowledge Centre · Water quality and aquarium stability
Cyanobacteria in aquariums: what red or blue-green slime actually is
A thin, slimy sheet spreading across substrate, decor or plant leaves — often blue-green, sometimes red, brown or nearly black — is usually cyanobacteria, commonly called "blue-green algae" or "red slime" despite not actually being algae at all. It's a bacterium, which is part of why algae-specific treatments often don't touch it and why scraping it off tends to bring only temporary relief. This guide covers what it actually is, what typically causes it, and the safer, more effective removal approach compared to reaching for the first product on a shelf.
At a glance
What it is
A photosynthetic bacterium, not a true algae, despite the common "blue-green algae" name.
Usual cause
A combination of low water flow, excess nutrients and accumulated organic waste, not one single factor.
What this page won't do
Recommend an antibiotic dose or promise a one-step fix — this is a persistent organism that needs the underlying cause addressed.
Understanding cyanobacteria
Cyanobacteria are photosynthetic bacteria, commonly known in the hobby as "blue-green algae" or, in its red form, "red slime algae," despite not being algae at all[1]. In aquariums they typically appear as a thin, slimy sheet or film — often peeling away in a single connected piece rather than crumbling — over substrate, decor, or slow-growing plant leaves, and can smell distinctly musty or "swampy" when disturbed.
Unlike true algae, which grows in individual filaments or cells that generally need to be physically disrupted, cyanobacteria forms a cohesive biofilm-like mat, and its bacterial biology means it doesn't respond the same way true algae does to some common algae-control approaches.
How it differs from true algae
| Feature | Cyanobacteria | True algae (e.g. hair or diatom-type) |
|---|---|---|
| Organism type | Bacteria | Algae (a different kingdom entirely) |
| Typical appearance | Thin, slimy sheet or mat, often blue-green or red | Individual filaments, threads, or a dusty/powdery coating |
| How it peels away | Often lifts off in one connected sheet | Usually crumbles or comes away in loose strands |
| Smell when disturbed | Often distinctly musty or "swampy" | Usually mild or absent |
| Responds to algae-specific treatments | Often poorly or not at all | Usually yes, depending on the product and algae type |
If what you're seeing is a fine, dusty brown coating, hair-like strands, or hard green spots rather than a slimy sheet, that's true algae rather than cyanobacteria — see our guide to algae in aquariums for the different types and how to control each one.
The toxin question, honestly
Some cyanobacteria species are well documented as capable of producing toxins in natural water bodies, and this is taken seriously as a public and environmental health issue in that context[2]. Whether the specific cyanobacteria commonly found in home aquariums produce meaningful toxin levels under typical tank conditions is genuinely not well established, and we don't want to overstate a risk the evidence doesn't clearly support. What is more consistently a concern is that a heavy, spreading bloom can smother substrate and plant surfaces and reduce oxygen exchange where it covers, which is worth treating seriously as a tank-health issue on its own, separate from the toxin question.
Likely causes
- Low or stagnant water flow. Cyanobacteria commonly establishes in slow-moving or dead-flow areas of a tank[3].
- Excess available nutrients. Elevated nitrate or phosphate from overfeeding, overstocking or infrequent water changes gives it more to work with.
- Accumulated organic waste. Detritus and uneaten food breaking down in low-flow areas is a commonly cited contributing factor[3].
- Lighting duration or intensity. Being photosynthetic, cyanobacteria can be encouraged by lighting that runs longer or more intensely than the tank's plants and livestock actually need.
It's rarely one single factor — a tank with good flow but excess nutrients, or good water changes but a dead-flow corner behind decor, can both provide enough of an opening for it to establish.
Immediate actions
Confirm and begin removal
-
Confirm it's cyanobacteria, not true algae
Use the comparison above — a slimy sheet that peels off in one piece and smells musty when disturbed is the more distinctive tell.
-
Manually remove what you can reach
Siphon it out directly during a water change rather than just scraping it loose into the tank, where it can resettle elsewhere.
Address the underlying cause
-
Improve flow to dead spots
Reposition a filter outlet or add a small powerhead to reach stagnant corners where it's establishing.
-
Increase water-change frequency temporarily
Reducing available nutrients directly, alongside checking you're not overfeeding.
-
Review lighting duration
Consider shortening it slightly if it's running longer than your plants and livestock need.
Why we're not recommending an antibiotic
What to check next
- Where in the tank is it concentrated — a specific low-flow area, or spread throughout?
- How long has it been since your last water change, and what's your current nitrate reading?
- Is lighting running longer than necessary for your plants and livestock?
- Is there an obvious source of excess organic waste — overfeeding, overstocking, or an overdue filter clean?
- Is it actually cyanobacteria, or could it be diatoms (fine, dusty brown coating) or true hair/thread algae instead?
When it may need more attention
Common mistakes
- Treating it as ordinary algae and reaching for an algae-specific product that isn't effective against a bacterium.
- Scraping it loose without siphoning it out, allowing it to resettle and re-establish elsewhere in the tank.
- Reaching for an antibiotic-based product before trying manual removal and correcting flow, nutrients and lighting.
- Addressing only the visible patch without improving the underlying low-flow or high-nutrient conditions that let it establish.
- Running lights for extended hours without reviewing whether that duration is actually needed.
Reducing recurrence
- Maintain good water flow throughout the tank, paying particular attention to corners and behind large decor.
- Keep nutrient input in check — avoid overfeeding and keep stocking appropriate to your filtration.
- Stick to a consistent water-change schedule rather than letting nitrate and organic waste accumulate.
- Run lighting for a duration appropriate to your plants and livestock, not simply as long as possible.
- Vacuum substrate regularly to prevent organic waste build-up in low-flow areas.
Frequently asked questions
Is cyanobacteria harmful to my fish?
Whether aquarium cyanobacteria produce meaningful amounts of toxin is genuinely not well established, and most home aquariums showing it don't report obvious fish harm from the bacteria itself. That said, a heavy cyanobacteria bloom can smother substrate and reduce oxygen exchange at the surfaces it covers, which is a real, separate concern worth treating seriously even without a confirmed toxin risk. We'd rather be upfront about this uncertainty than claim more confidence than the evidence supports.
Is cyanobacteria the same as algae?
No, despite being commonly called blue-green algae. Cyanobacteria are bacteria, not true algae, which is part of why algae-specific treatments and products don't reliably work against it and why it often needs a different removal approach. Getting the identification right first matters, since treating it like ordinary algae is a common, ineffective first response.
Why does cyanobacteria keep coming back after I scrape it off?
Because scraping or siphoning removes the visible sheet but doesn't address whatever combination of low flow, excess nutrients and organic waste let it establish in the first place. It's a genuinely persistent organism that tends to return quickly unless the underlying conditions actually change — improving flow, reducing nutrient input and increasing water-change frequency matter more for long-term control than repeated manual removal alone.
Should I use an antibiotic to treat cyanobacteria?
Some antibiotic-based products are marketed for this, but we're deliberately not recommending one here. Using an antibiotic for a cosmetic bacterial bloom raises genuine concerns around antibiotic resistance and potential effects on your tank's beneficial nitrifying bacteria, and isn't something to reach for before trying manual removal and correcting the underlying causes covered in this guide. If you're considering it, that's a decision worth researching carefully or discussing with a specialist rather than following a generic recommendation.
How AquaPulse Water Check can help
A cyanobacteria bloom looks sudden but almost never is — it's what nitrate creeping up week after week eventually looks like once it crosses a threshold. The creep itself is invisible; a clean glass on a Tuesday doesn't feel any different from a clean glass the Tuesday before. Logging your nitrate readings in AquaPulse alongside your maintenance turns that creep into something you can actually watch happen, so the threshold-crossing is something you see coming rather than something you scrub off and then wonder about.
Water Check works from the information you enter — it can't identify what's growing in your tank or confirm cyanobacteria specifically, and isn't a substitute for testing your water with a reliable kit. Get AquaPulse on the App Store or Google Play to try it, or see the Water Check feature page for more detail.
Sources and editorial information
- Published
- 30 July 2026
- Last reviewed
- 30 July 2026
- Written and maintained by
- AquaPulse (Valeon Labs Ltd)
This guide draws on a peer-reviewed review of cyanotoxin risk in natural water bodies for background on toxin production generally, and specialist aquarium-keeping sources for practical aquarium-specific causes and removal detail, since dedicated peer-reviewed research on cyanobacteria specifically within home aquariums is limited. We've been explicit that aquarium-specific toxin risk isn't well established rather than overstating it either way, and we deliberately avoid recommending a specific antibiotic product or dose. We'll update this page if we find something we've got wrong.
Good to know: This guide provides general informational guidance only. It is not a veterinary service and does not replace specialist or aquatic health advice for serious livestock health issues.
- [1] FishLab — "Blue-Green Algae Is Not Algae: How To Kill It And Keep It Gone" Specialist hobbyist source. Used for cyanobacteria's classification as bacteria rather than true algae, typical aquarium appearance, and why algae-specific treatments often don't work against it. Not a scientific authority.
- [2] "Understanding the Risks of Diffusion of Cyanobacteria Toxins in Rivers, Lakes, and Potable Water," peer-reviewed review, PMC10535532 Peer-reviewed review article. Used for background on cyanotoxin production being a documented risk in natural water bodies generally — not aquarium-specific, and cited here to support the honest "not well established in home aquariums specifically" framing rather than as direct evidence of aquarium risk.
- [3] Aquarium Co-Op — "How to Get Rid of Blue-Green Algae in Freshwater Aquariums" Specialist retailer/hobbyist source, already used in a similar role elsewhere on this site. Used for common aquarium-specific causes (low flow, nutrient buildup, organic waste) and manual-removal practicalities. Not a scientific authority.
What we deliberately left out: any specific antibiotic product name or dosage, for the reasons explained above; and a definitive answer on aquarium-specific toxin risk, since we found this genuinely unresolved in the sources available rather than a settled question we could report confidently either way.