Cyanobacteria in the aquarium — identification and treatment

Cyanobacteria in the aquarium — identification and treatment

Red slime spreads across the sandbed overnight, expands in light and retreats in darkness. The hobbyist finds ten different pieces of advice online: lower nutrients, raise nutrients, increase flow, change the lights, use an antibiotic, don’t use an antibiotic. Some succeed, others don’t — and the difference is often whether the hobbyist understood what they were fighting.

Cyanobacteria are one of the most common problems in reef aquariums, yet one of the most misunderstood. They are not algae. There is no single cause. And antibiotics are not the solution — they are the last resort, and they come at a cost.

What cyanobacteria are — and what they are not

Cyanobacteria (Cyanobacteria) are prokaryotes — bacteria without a nucleus. They are the inventors of photosynthesis: around 2.7 billion years ago, cyanobacteria produced Earth’s oxygen and made multicellular life possible. On natural reefs they are an essential part of the ecosystem — nitrogen fixers, members of endolithic communities, and even components of the coral holobiont.

In an aquarium, the problem is not the presence of cyanobacteria as such — they are always there. The problem is when conditions give them a competitive advantage over the rest of the microbial community, and a single group begins to dominate.

The most common forms in aquariums:

• Red or violet slime mat — the most common, typically on the sandbed or rock surfaces, expands in light and retreats in darkness
• Green slime layer — more common in older tanks, can resemble green algae but feels slicker
• Blue-green or black mat — rarer, may appear on rock surfaces

Cyanobacteria are gram-negative bacteria. This has practical relevance when antibiotics are discussed later.

Identification: cyanobacteria or something else?

Before treatment, identify the problem. Red or brown slime can also be dinoflagellates, diatoms or other microorganisms — and their treatment strategies differ significantly.

The shake test

The simplest field test: take a water sample in a bottle, add a little slime and shake vigorously for 10–15 seconds.

• Cyanobacteria: breaks apart, water turns turbid green or blue-green
• Dinoflagellates: withstands shaking, forms a uniform suspension without a clear colour change
• Diatoms: breaks apart, water turns brownish

Visual characteristics

The shake test combined with a visual assessment is usually sufficient for identification. If you suspect dinoflagellates, consult the dinoflagellates article — their treatment strategy is entirely different.

Why cyanobacteria take over — a multi-factor problem

Cyanobacteria do not need a single trigger. They thrive when several things are simultaneously out of balance. The most important factors:

1. Flow deficiencies — the single most common cause

Cyanobacteria favour areas where flow is weak. They need a calm surface to form a biofilm. Flow deficiencies are the single most common cause of cyanobacteria problems — ahead of nutrients, ahead of light.

In practice, this typically shows up:

• On sandflats behind the front glass
• In the shadows and pockets of rocks
• At the lower back of the tank
• In the sump or technical areas where water stagnates

2. Nutrients — a more complex picture than assumed

Common misconception: “cyanobacteria require high nutrients.” Reality is more complex.

Cyanobacteria grow at both high and low nutrient levels — for different reasons:

High nutrients + weak flow: Organic load feeds them directly. This is the classic scenario in a poorly maintained tank.

Low or zero nutrients: Cyanobacteria can fix nitrogen from the air (nitrogen fixation), so they do not need nitrate from the water. When nitrate and phosphate are very low, competing microbes suffer from nutrient shortage while cyanobacteria manage fine. This explains why aggressive carbon dosing or GFO use can trigger a cyanobacteria outbreak — nutrients drop so low that competing microbial populations weaken.

Ammonia vs. nitrate: Cyanobacteria prefer ammonia roughly ten-fold over nitrate. When ammonia is present, they shut down their nitrate uptake pathways and take ammonia for as long as it is available. This explains why ammonia dosing can theoretically worsen cyanobacteria problems — though in practice a sufficient nitrification bacterial population neutralises ammonia quickly.

3. Organic load — detritus and decaying matter

Detritus — decaying organic matter in the sand, rock pockets and filter areas — is a preferred environment for cyanobacteria. They do not so much “eat” detritus directly, but decaying matter creates anaerobic micro-pockets and releases ammonium that cyanobacteria readily use.

Practical sign: if cyanobacteria always grow in the same spots, check whether detritus has accumulated there.

4. Light

Cyanobacteria are photosynthetic and grow in light. They thrive especially at low light intensities — they do not need much. This explains why they are often found on the shaded surfaces of rocks and in poorly lit areas.

Light spectrum can also play a role: some strains respond more strongly to certain wavelengths. For this reason, shortening the photoperiod is often an ineffective treatment — it weakens corals more than cyanobacteria.

5. Trace element deficiencies — an ICP-data observation

Fauna Marin’s ICP database includes a cyano prediction function based on a large reef aquarium ICP dataset. The data consistently links cyanobacteria outbreaks to four factors:

• Iodine below 0.05 mg/l
• Fluoride below 1.0 mg/l
• Bromine below 70 mg/l
• Nitrate/phosphate ratio clearly above 100

The mechanism has not yet been explained in the research literature. Iodine and bromine are known antimicrobial compounds in reef environments — their deficiency may weaken the tank’s natural biological defences. The fluoride link is less clear.

Practical implication: if cyanobacteria recur and no root cause is found in flow or nutrients, an ICP analysis is worthwhile — check iodine, fluoride and bromine levels in particular. A GFO reactor removes fluoride and iodine efficiently; active GFO use may be an underlying cause of their decline.

6. Immature or imbalanced microbiome

In new tanks, cyanobacteria problems are common — the microbiome has not yet developed to compete against them. The same can happen in an established tank after chemical treatment, massive water change series or other disruptions.

Treatment strategy: address the cause before the symptom

Cyanobacteria treatment only works when it targets the root cause. Treating the symptom without correcting the cause leads to repeated outbreaks.

Step 1: Mechanical removal

Remove visible cyanobacteria mass immediately by siphoning. This:

• Removes the biomass and its contained nutrients from the tank
• Gives competing microbes space
• Speeds tank recovery

Practical approach: run the siphon through a filter sock — cyanobacteria stays in the sock, clean water returns to the tank. Repeat daily as needed.

Step 2: Flow optimisation

Inspect the tank carefully and find dead zones. Reposition or add circulation pumps. The goal is to have no spot in the tank where water is stagnant.

Check especially:

• Sandflats behind the front glass and at the sides
• Under-surfaces of rocks
• Corner areas of the tank

Step 3: Nutrient diagnosis

Measure nitrate and phosphate. Based on the result:

If both are near zero (below 1 mg/l NO₃, below 0.03 mg/l PO₄): Nutrient deficiency is a likely contributing factor. Increase feeding moderately, or consider dosing nitrate to a target of 2–5 mg/l. This strengthens the competing microbiome without directly “feeding” cyanobacteria.

If nutrients are high (NO₃ above 20 mg/l, PO₄ above 0.15 mg/l): Organic load is likely the primary cause. Reduce feeding, check filtration, increase water changes.

If nutrients are at a moderate level: Flow and detritus are the most likely causes. Continue with mechanical removal and flow improvement.

Step 4: Detritus removal

Brush rocks, remove detritus from the sandbed thoroughly. This is labour-intensive but often decisive. Detritus removal takes away both the food source and the micro-pockets that cyanobacteria favour.

Step 5: Strengthening bacterial competition

Add live microbial cultures — bacterial cultures such as Nyos BACTO PLUS, Fauna Marin Bacto Fill, KZ Cyano Clean or equivalent. These introduce heterotrophic bacteria that compete with cyanobacteria on surfaces and for nutrients.

This is not a quick fix — it takes weeks before the competing microbiome becomes established. Patience is essential.

Chemical treatments — when and how

Chemical treatments are the last resort, not the first. Use them only when mechanical and biological measures have not produced results over several weeks, or when cyanobacteria threaten to suffocate corals.

Chemiclean and similar products

Chemiclean (Boyd Enterprises) is the most widely used product. It is an antibiotic targeted at gram-positive bacteria — paradoxically, since cyanobacteria are gram-negative. Despite this, it works against cyanobacteria by a mechanism that is not fully understood.

Practical instructions:

• Follow the dosing instructions precisely — do not overdose
• Add an airstone to ensure oxygenation during treatment
• Perform a large water change 24–48 hours after treatment
• Keep the skimmer running throughout

Chemiclean does not fix the root cause. If conditions remain the same, cyanobacteria will return.

Antibiotics — what not to do

Erythromycin and other broad-spectrum antibiotics have circulated in the hobby as a cyanobacteria treatment. This approach is problematic for several reasons:

Microbiome destruction: Broad-spectrum antibiotics do not only target cyanobacteria — they damage the entire tank bacterial population, including nitrifying bacteria and other beneficial microbes. After treatment, the tank can experience an ammonia spike.

Toxin release: Under antibiotic stress, cyanobacteria can release internal toxins into the water column. This is a documented phenomenon in research on freshwater cyanobacteria — the mechanism is the same in a reef aquarium.

Resistance: Antibiotic exposure drives resistance. Antibiotic-resistance-gene-carrying bacteria circulating in a reef aquarium are not merely a theoretical concern.

Ciprofloxacin and other broad-spectrum antibiotics are unnecessary for external treatment. If chemical treatment is needed, Chemiclean is the better option.

Long-term prevention

The best treatment for cyanobacteria is prevention. A stable, biologically diverse tank gives cyanobacteria little opportunity to establish.

Design flow first: Ensure already at the planning stage that no dead zones will form. Positioning circulation pumps is cheaper to correct before the tank is full.

Regular detritus removal: Weekly detritus removal from the sandbed and rock brushing ensures that decaying pockets do not develop.

Stable nutrients: Stay away from both zero nutrients and very high nutrients. NO₃ 2–10 mg/l and PO₄ 0.03–0.08 mg/l is a biologically functional range where competing microbes thrive.

Living microbiome: Live rock, regular bacterial culture additions and plankton dosing maintain a diverse microbial community that naturally keeps cyanobacteria in check.

New tank situation: In a young tank, cyanobacteria are almost expected — the microbiome is not yet mature. Patience and correct conditions are the best medicine here. Do not react too aggressively with chemicals in a young tank.

Summary — three decision points

1. Identify first. The shake test distinguishes cyanobacteria from dinoflagellates. A wrong diagnosis leads to wrong treatment.

2. Fix the cause before the symptom. Flow, detritus and nutrient diagnosis before chemicals. Chemiclean without correcting the root cause is a temporary solution.

3. Build the microbiome for the long term. Cyanobacteria do not take over a tank by force — they fill the void that forms when competing microbes have weakened. A biologically diverse tank is the most durable defence.

References

Peer-reviewed research

• Lesser, M. P., Mazel, C. H., Gorbunov, M. Y. & Falkowski, P. G. (2004). Discovery of symbiotic nitrogen-fixing cyanobacteria in corals. Science, 305(5686), 997–1000. https://doi.org/10.1126/science.1099128
• Wu, J., Shang, L., Lin, Y., Mao, Y., Li, H. & Song, L. (2020). Resistance of cyanobacteria Microcystis aeruginosa to erythromycin with multiple exposure. Chemosphere, 249, 126143.
• Paerl, H. W. & Huisman, J. (2008). Blooms like it hot. Science, 320(5872), 57–58.
• Kayanne, H., Hirota, M., Yamamuro, M. & Koike, I. (2005). Nitrogen fixation of filamentous cyanobacteria in a coral reef measured using three different methods. Coral Reefs, 24, 197–200.

Hobbyist literature and brand documentation

• BRStv / Bulk Reef Supply (2021). Get Rid Of Cyanobacteria Or Red Slime Algae Forever! bulkreefsupply.com.
• BRStv / Bulk Reef Supply (2024). Defeat Cyanobacteria — Give Red Slime the Boot With the Right Tools! bulkreefsupply.com.
• Manta Systems (2025). The Ultimate Guide to Understanding and Battling Cyanobacteria. mantasystems.net.
• Coralaxy (2024). How to fight cyanobacteria in reef tanks. coralaxy.de/en/blogs.

Books and textbooks

• Delbeek, J. C. & Sprung, J. (2005). The Reef Aquarium: Science, Art, and Technology. Two Little Fishes, Miami Gardens.
• Brock, T. D. & Madigan, M. T. (2015). Biology of Microorganisms, 14th ed. Pearson, Upper Saddle River.