Cyanobacteria in Reef Tank: Why It Keeps Coming
Quick Summary
Cyanobacteria (cyano) is a photosynthetic bacterium that forms red, maroon, or dark green slimy mats across sand, rock, and equipment in reef tanks. It is not algae, though it is often grouped with algae problems. Cyano is one of the most common nuisances in reef keeping and one of the most frequently mismanaged.
Here is what matters right away:
- Cyano is a symptom, not the disease. It appears because conditions in your tank favour it. Killing the cyano without fixing the conditions guarantees it returns.
- Flow is almost always involved. Cyano dominates in areas where water movement is weak or absent. Dead spots are cyano nurseries.
- Chemical treatments work fast but fail long-term. Products like Chemiclean eliminate visible cyano in 48 hours, but it grows back within weeks if the root cause remains.
- Cyano is fixable. Once you understand what drives it, the solution is straightforward and permanent.
The Pattern Most Reefers Miss
When cyano appears, most aquarists go straight to nutrients. They test nitrate and phosphate, find one slightly elevated, and start running GFO or increasing water changes. The cyano might thin out briefly, but it returns. They dose Chemiclean and it disappears overnight. Two weeks later, it is back.
This cycle repeats because the reefer is treating cyano like algae. Algae responds to nutrient reduction. Cyano does not, at least not in the way most people expect.
Cyanobacteria is not primarily a nutrient problem. It is a flow and dissolved organics problem. The reefers who solve cyano permanently are the ones who stop chasing nitrate numbers and start examining where the cyano is growing. The location tells you more than any test kit.
If cyano is concentrated in specific areas rather than coating the entire tank uniformly, flow is almost certainly the primary driver. If it is widespread, dissolved organics and deferred maintenance are the more likely cause.
What Cyanobacteria Actually Is
Cyanobacteria are among the oldest organisms on Earth. They are prokaryotic (bacteria, not algae), and they predate the existence of coral, fish, and every other organism in your reef tank by billions of years. They are extraordinarily adaptable, which is why they are so difficult to outcompete.
In your tank, cyano forms visible colonies that look like slimy sheets or mats. The colour comes from pigments used for photosynthesis: phycocyanin (blue-green) and phycoerythrin (red). Reef tank cyano is most commonly red or dark maroon, though green and black varieties exist.
You can identify cyano by its behaviour. It peels off surfaces in coherent sheets or ribbons rather than breaking apart like diatoms. It has a distinctive musty, earthy smell. It traps gas bubbles beneath its mat as it photosynthesizes, sometimes causing sections to lift off the sandbed and float.
If you have ever peeled a sheet of cyano off your sand and noticed the sand underneath is dark grey or black, you have seen another key characteristic: cyano mats create anaerobic zones beneath them where oxygen is depleted and hydrogen sulfide forms. That dark discoloration and rotten egg smell is a direct consequence of cyano smothering the sandbed.
Why Cyano Appears in Reef Tanks
Cyano thrives where other organisms struggle. It is not competing in optimal conditions. It is exploiting suboptimal ones. Every cyano outbreak maps back to one or more of these root causes.
Dead Spots in Flow
This is the single most common cause of cyano in reef tanks. Areas where water movement is weak or absent allow detritus to settle, dissolved organics to accumulate, and oxygen levels to drop. These are the exact conditions cyano needs.
Most reefers notice cyano first in predictable locations:
- Behind rock structures where flow cannot reach
- In corners of the tank furthest from powerheads
- Along the back glass where circulation is weakest
- On the sandbed in areas shielded by the aquascape
If your cyano is concentrated in specific spots rather than covering the entire tank, flow is your primary issue. The solution is mechanical, not chemical.
Elevated Dissolved Organics
Dissolved organic compounds (DOCs) are the invisible fuel source that feeds cyano. Unlike nitrate and phosphate, which are measured by standard test kits, dissolved organics are not routinely tested in most reef tanks. They accumulate from fish waste, uneaten food, coral mucus, and decomposing organic matter.
In most tanks, the protein skimmer is the primary tool for removing dissolved organics. When the skimmer is underperforming, turned off, or undersized, DOCs build up in the water column. Cyano responds to this faster than most other organisms because it can metabolise a wide range of organic compounds.
Signs that dissolved organics are driving your cyano:
- Skimmer is not producing dark, consistent skimmate
- Filter socks or media have not been changed recently
- The tank has a slight yellowish tint to the water
- Activated carbon has not been replaced in over a month
- The outbreak is widespread rather than localised
New Tank Syndrome
Cyano is a predictable part of the new tank ugly phase. During the first 2 to 6 months, the biological filtration is immature, nutrient cycling is inconsistent, and microbial populations have not yet stabilised. Cyano exploits this transitional period because it can fix nitrogen from dissolved gas, giving it an advantage when ammonia and nitrate cycling is still erratic.
In most new tanks, cyano appears after the initial diatom bloom fades (around weeks 4 to 8) and persists until the tank's biology matures enough to outcompete it. If your tank is less than six months old and cyano is your main complaint, time and consistent maintenance are usually the only intervention needed.
Old Substrate and Detritus Buildup
In mature tanks, cyano sometimes appears after years of clean operation. This often puzzles experienced reefers who have not changed their maintenance routine.
In practice, this pattern usually traces back to accumulated detritus. Sand beds compact over time. Detritus builds up in the rock structure, beneath equipment, and in the sump. The organic load in the system gradually increases even if the visible surfaces look clean. At some point, the dissolved organics from this hidden waste reach a level where cyano can establish.
If your mature tank suddenly develops cyano, look beneath the surface. Stir the sandbed gently in the affected area and observe what comes out. If a cloud of detritus billows up, accumulated waste is your answer.
Lighting Schedule Changes
Cyano is photosynthetic and responds to light availability. Extending your photoperiod, increasing intensity, or even shifting the light schedule can create conditions where cyano receives more usable light energy, particularly in areas that were previously too shaded to support growth.
This is a secondary factor rather than a primary cause. Light changes alone rarely trigger cyano, but they can push a borderline system over the threshold if flow or organics are already marginal.
How to Eliminate Cyanobacteria Permanently
Beating cyano requires addressing root causes. The visible mats are the symptom. The real problem is the combination of weak flow, elevated organics, and insufficient competition.
Fix Flow First
This is the highest-impact intervention for most cyano outbreaks. Before touching water chemistry, dosing anything, or buying a product, evaluate and improve water movement.
Steps to fix flow:
- Map your dead spots. Drop a pinch of food or a tiny amount of dye near the cyano growth and watch where it goes (or does not go). Dead spots become obvious.
- Reposition powerheads. Aim for randomised, turbulent flow across all surfaces. Avoid laminar streams that create strong flow in one area and dead zones in another.
- Add a powerhead if needed. Most reef tanks benefit from 20 to 50 times total turnover per hour from flow pumps. If your total flow is below this, adding a wavemaker can eliminate dead spots.
- Adjust the aquascape. Rock walls that block flow create dead zones behind them. Opening up the aquascape to allow water to pass through and behind structures solves flow problems at their source.
In many cases, fixing flow alone eliminates cyano within 2 to 3 weeks without any other intervention.
Optimise Nutrient Export
With flow addressed, the next step is ensuring your system is exporting dissolved organics effectively.
Key actions:
- Clean and tune the protein skimmer. Remove the skimmer, clean the neck, body, and pump impeller thoroughly. Adjust the water level in the skimmer body so it produces wet, dark skimmate consistently. A clean skimmer pulls noticeably more than a neglected one.
- Replace activated carbon. Run fresh high-quality carbon and replace it every 3 to 4 weeks. Carbon removes dissolved organics that fuel cyano.
- Change filter socks every 2 to 3 days. A saturated filter sock becomes a compost bin, leaching organics back into the water instead of removing them.
- Vacuum the sandbed. During water changes, use a siphon to vacuum the top layer of sand, particularly in areas where cyano has been growing. This removes the detritus and organic layer that feeds the next bloom.
- Clean the sump. If detritus has accumulated in the sump chambers, baffles, or around equipment, clean it during a water change. The sump is often the most neglected nutrient source in the system.
Manual Removal
While you fix the underlying causes, manually removing cyano reduces the organism's biomass and the nutrients locked within it.
Effective removal techniques:
- Siphon during water changes. Place the siphon directly on cyano mats and vacuum them out. Cyano peels up easily in sheets, making siphon removal straightforward. This is the most effective manual method because the cyano and the water leave the system entirely.
- Turkey baster. Blast cyano off rocks into the water column and let your filter sock or skimmer capture it. Less effective than siphoning but useful between water changes.
- Do not just peel and leave. Peeling cyano off and leaving it floating in the tank releases its stored nutrients back into the water. Always export it from the system.
When to Use Chemical Treatment
Chemical treatments like Chemiclean (erythromycin-based) are effective at killing cyanobacteria quickly. A single dose typically eliminates visible cyano within 24 to 48 hours.
However, chemical treatment should be the last step, not the first. Use it only after you have:
- Improved flow to eliminate dead spots
- Optimised your skimmer and export processes
- Cleaned accumulated detritus from the sand, rocks, and sump
If you use Chemiclean without fixing the root causes, the cyano returns within 1 to 3 weeks because the conditions that grew it are unchanged. If you fix the root causes first and then treat chemically, the cyano has nothing to come back to.
Important notes for chemical treatment:
- Remove activated carbon before dosing (carbon absorbs the medication)
- Increase aeration during treatment (cyano die-off consumes oxygen)
- Run the protein skimmer with the collection cup removed (to maximise surface agitation and oxygenation without removing the medication)
- Perform a 20 percent water change 48 hours after treatment
- Replace carbon after the water change
Cyano vs. Dinoflagellates: Getting the Diagnosis Right
Cyano and dinoflagellates can look similar at first glance, but the treatment approaches are opposite. Misidentifying dinos as cyano and treating with nutrient reduction makes the dino problem dramatically worse.
Here is how to tell them apart:
- Cyano peels in sheets, has a musty smell, and favours low-flow areas. It responds to flow improvements and chemical treatment.
- Dinoflagellates are stringy and slimy, produce visible bubbles trapped in strands during the day, and tend to appear in ultra-low nutrient conditions. They do not peel in sheets and do not respond to cyano treatments.
If your brown or red slime produces visible gas bubbles trapped within stringy mucus during the light period and recedes dramatically at night only to return each morning, you likely have dinoflagellates, not cyano. See the dinoflagellates guide for the correct treatment approach.
Advanced: Why Cyanobacteria Can Fix Nitrogen
One of the reasons cyano is so persistent is its ability to fix atmospheric nitrogen. Most organisms in your reef tank depend on ammonia and nitrate as nitrogen sources. Cyanobacteria can convert dissolved nitrogen gas (N2) directly into usable ammonia through an enzyme called nitrogenase.
This gives cyano a survival advantage in conditions where other organisms are nitrogen-limited. Even if your nitrate reads zero, cyano can source its own nitrogen from dissolved gas in the water. This is why nutrient reduction alone does not reliably eliminate cyano the way it eliminates green hair algae.
This nitrogen-fixing ability also explains why cyano often appears in new tanks before the nitrogen cycle is fully established. While nitrifying bacteria are still building populations to process ammonia and nitrite, cyano is already self-sufficient and can colonise surfaces without waiting for the cycle to mature.
Advanced: The Anaerobic Zone Problem
Cyano mats create a unique problem beneath their surface. The dense mat blocks water exchange and oxygen diffusion, creating an anaerobic zone between the mat and the substrate. In this oxygen-free environment, sulphate-reducing bacteria produce hydrogen sulfide (H2S), a toxic gas with a distinctive rotten egg smell.
In most cases, the H2S produced under small cyano mats dissipates harmlessly into the water column when the mat is disturbed. But in tanks with extensive cyano coverage on deep sand beds, the accumulated H2S can reach levels that are toxic to nearby invertebrates and fish.
This is why large cyano mats on sand beds should be removed carefully. Peeling back a large mat suddenly releases trapped H2S. Siphon the mat out gradually during a water change rather than ripping it off and allowing the gas to disperse into the tank all at once.
This anaerobic effect is also why cyano-covered sand turns dark grey or black. The dark colour comes from iron sulfide formed by the reaction between H2S and iron in the substrate. When the cyano is removed and oxygen reaches the sand again, the colour gradually returns to normal as the sulfide oxidises.
Common Myths
"Cyano means your tank has too many nutrients." Cyano is primarily driven by dissolved organics and poor flow, not elevated nitrate or phosphate. Many tanks with detectable nutrients never develop cyano because their flow and export are adequate. Conversely, tanks with low nitrate and phosphate can develop cyano if flow is poor and organics accumulate.
"Chemiclean fixes cyano." Chemiclean kills the existing cyano effectively. It does not fix the conditions that grew it. Without addressing flow and organics, cyano returns within weeks of treatment. Chemiclean is a reset button, not a solution.
"Reduce your lights to get rid of cyano." Reducing light slows cyano growth but also harms corals. If you eliminate the flow and organics issues driving the outbreak, cyano disappears at normal lighting levels. Light reduction is a band-aid that penalises your entire reef.
"Cyano only happens in new tanks." New tanks are prone to cyano, but mature tanks develop it too. Accumulated detritus, failing equipment, or changes to flow patterns can trigger cyano in systems that ran clean for years.
"More snails and hermits will eat cyano." Almost no common clean-up crew members eat cyanobacteria reliably. Snails and hermits prefer algae. Cyano has a chemical taste that deters most grazers. Do not overstock your clean-up crew expecting them to solve a cyano problem.
FAQ
How long does it take to get rid of cyanobacteria? With flow improvements and optimised export, most cyano outbreaks resolve in 2 to 4 weeks. Chemical treatment speeds visible elimination to 48 hours, but the underlying fixes still need 2 to 3 weeks to prevent recurrence.
Is cyanobacteria harmful to coral? Cyano mats that grow directly over coral tissue can smother and kill the covered area by blocking light and depleting oxygen. Cyano growing on nearby sand or rock does not directly harm coral, but the conditions that drive cyano (poor flow, elevated organics) are also suboptimal for coral health.
Can I just siphon cyano out forever? You can manage cyano with regular siphoning, but it will continue growing as long as the conditions favour it. Siphoning removes biomass but does not fix flow or organic accumulation. Address the root cause and the need for siphoning disappears.
Why does cyano keep coming back after Chemiclean? Because Chemiclean kills the bacteria but does not change the environment. The dead spots, accumulated organics, and weak flow that supported the original outbreak support the next one. Fix those conditions before or alongside chemical treatment.
Should I remove my sandbed to get rid of cyano? Removing the sandbed eliminates the most visible cyano surface, but cyano can grow on rock, glass, and equipment equally well. A bare-bottom tank with poor flow and elevated organics will still develop cyano. Fix the system, not the substrate.
Does cyano produce toxins? Some cyanobacteria species produce toxins (cyanotoxins), but the species common in reef aquariums are generally not acutely toxic. The primary risk is from hydrogen sulfide produced under large mats and from oxygen depletion during large die-offs (either natural or chemical treatment-induced). Maintain aeration during any treatment.
My cyano is green, not red. Is it still cyano? Yes. Cyanobacteria contain multiple pigments. Red and maroon are most common in reef tanks, but green and dark blue-green varieties exist. The identifying behaviour is the same: slimy mats that peel in sheets with a musty smell. If it sheets off surfaces and smells earthy, it is cyano regardless of colour.