Cyanobacteria in Reef Tanks: How to Control Red Slime Safely

Cyanobacteria is often called red slime algae, but it is not true algae. It is photosynthetic bacteria growing as a visible colony.

That distinction matters because the usual “algae problem” reaction can make it worse. Turning the lights off may hide it temporarily. Driving phosphate and nitrate to zero may remove competitors. A chemical treatment may clear the mat while leaving the same dead spot, waste build-up and unstable nutrient pattern underneath.

My view is: remove the visible cyano, then fix the conditions that let it return. Do not use a bottle as a substitute for diagnosis.

Quick answer

A practical cyano response is:

1. Confirm that the growth forms a soft, cohesive, slimy mat.
2. Siphon out as much as possible instead of stirring it through the tank.
3. Clean detritus traps such as dirty filter socks, sump corners and the area behind rockwork.
4. Improve circulation around the affected surface without blasting coral.
5. Test nitrate and phosphate and keep both measurable and stable.
6. Review feeding, source water, skimming and mechanical filtration.
7. Shorten an excessive photoperiod only as support, not as the main cure.
8. Wait and observe after each change instead of changing five systems at once.
9. Use a cyanobacteria treatment only when the basic causes have been addressed and oxygenation can be protected.

What cyanobacteria looks like

Cyano commonly appears as:

• red, burgundy, purple, brown, green or nearly black slime;
• a thin sheet covering sand, rock, frag racks or equipment;
• a mat that lifts or peels in pieces;
• bubbles trapped in the film during the light period;
• strong growth in areas where detritus settles;
• a coating that returns to the same place after removal.

The colour is not a diagnosis. Red is common, but some cyanobacteria is dark brown or green.

The texture is more useful. Cyano usually holds together like a soft sheet when siphoned. Diatoms behave more like loose dust. Dinoflagellates may form strings, mucus or a dusty coating depending on the type.

Cyano, dino or diatoms?

Feature	Cyanobacteria	Dinoflagellates	Diatoms
Texture	Slimy, cohesive sheet	Dust, mucus or strings	Fine dusty coating
Removal	Often siphons or peels in mats	Disperses; may return quickly	Wipes away easily
Bubbles	Common in established mats	Common in some slimy types	Usually limited
Daily behaviour	Often remains visible at night	Some types reduce after lights out	Usually remains
Common location	Sand and low-flow surfaces	Sand, rock, glass or coral	New surfaces and sand
Main response	Remove mat and correct flow, waste and balance	Identify behaviour and choose matched treatment	Allow maturation and control silicate/source inputs if persistent

When the appearance is ambiguous and the tank is deteriorating, microscopy is more useful than guessing from colour.

Why cyano appears

Cyano rarely has one cause. Most outbreaks are a stack of smaller problems.

Detritus and dissolved organics

Uneaten food, fish waste, dead algae and dirty mechanical media release nutrients and organic compounds. A sheltered surface with constant waste settlement is ideal for a mat to establish.

Poor circulation

Low-flow zones do not create cyanobacteria by themselves, but they allow waste to settle and the mat to remain undisturbed. Behind rockwork, around frag racks and along protected sand edges are common starting points.

Nutrient imbalance

Cyano can appear when nitrate and phosphate are high. It can also appear when one nutrient is undetectable while the other remains available.

This is why “reduce nutrients” is incomplete advice. The goal is not zero. The goal is a stable system where coral, algae, bacteria and filtration compete normally.

Immature or disrupted biology

New tanks go through microbial succession. Mature tanks can also develop cyano after:

• deep cleaning;
• antibiotic treatment;
• sudden carbon dosing;
• a large livestock loss;
• changing filtration or lighting;
• replacing major rock or substrate;
• driving nutrients down too quickly.

Lighting

Cyano uses light, so excessive intensity or photoperiod can increase growth. Lighting is usually an amplifier rather than the only cause.

If the same dead spot remains dirty and nutrient balance remains unstable, the mat will often return when the lights resume.

Does cyanobacteria harm coral or fish?

A small patch is usually more warning sign than emergency. A heavy mat can become harmful.

It may:

• cover coral tissue and block light;
• irritate LPS or zoanthids around the base;
• trap detritus against coral skeleton;
• reduce flow at the tissue surface;
• consume oxygen at night in dense growth;
• cover sand used by invertebrates;
• indicate broader instability that stresses livestock.

Cyano is not usually a direct fish predator. The main risk is the environment created by a large bloom and the treatment used against it.

Step-by-step control plan

Step 1: Record the tank before changing it

Measure and write down:

• nitrate;
• phosphate;
• salinity;
• temperature;
• alkalinity;
• recent feeding changes;
• recent water changes or additive use;
• the exact locations where cyano grows.

A photo taken at the same time each day is useful. It shows whether the outbreak is actually improving instead of relying on memory.

Step 2: Siphon the mat

Physical removal provides immediate relief and exports material from the system.

Use a narrow hose to lift the sheet from sand and rock. Remove it from the aquarium instead of brushing it into small fragments.

If water is returned through a filter sock, use sufficiently fine clean media and remove it immediately afterwards. Do not let the captured slime decay in the sump.

Step 3: Clean the real waste traps

Look beyond the visible patch:

• dirty filter socks or floss;
• detritus under the rock edge;
• the back chamber of an all-in-one tank;
• sump corners;
• blocked pump guards;
• uneaten food beneath feeding spots;
• dead macroalgae in a refugium.

Cleaning these areas is more valuable than repeatedly scrubbing the same red patch.

Step 4: Improve flow intelligently

Reposition a pump so water moves across the affected surface and carries debris towards filtration.

Do not aim a concentrated jet directly at coral tissue or create a sandstorm. The target is broader circulation and less settlement, not maximum pump speed.

After coral grows, old pump positions may no longer work. A tank can develop new dead spots without any equipment failing.

Step 5: Stabilise nutrients

If nitrate or phosphate is undetectable, do not continue stripping the water harder. Reduce or pause the export method responsible and restore measurable levels slowly.

If both are elevated, reduce the input and improve export gradually:

• feed appropriate portions;
• remove uneaten food;
• service the skimmer;
• change mechanical media on time;
• use clean source water;
• harvest macroalgae;
• perform measured water changes.

Avoid dramatic swings. Cyano often benefits from instability as much as from excess.

Step 6: Review the light schedule

A modest temporary reduction can slow growth and make removal easier.

Do not treat darkness as proof that the problem is solved. When light returns, cyano can return if the nutrient, flow and waste conditions remain unchanged.

Protect coral from sudden large intensity changes. Gradual adjustment is safer than repeatedly switching between full light and blackout.

Chemical treatments: when to consider them

A purpose-made cyanobacteria treatment can be effective against visible mats. It should be a controlled intervention, not normal maintenance.

Before using one:

• confirm that the problem is likely cyanobacteria;
• manually remove as much as possible;
• address flow and waste accumulation;
• read the current manufacturer instructions;
• provide strong aeration and gas exchange;
• understand whether skimmer, UV, ozone or carbon must be adjusted;
• calculate the true water volume rather than tank label volume;
• prepare for the instructed water change and media replacement;
• monitor fish and coral throughout treatment.

The main risks are oxygen depletion, skimmer overflow, stress to biological filtration and rapid decay of killed biomass.

I would not use repeated antibiotic-style treatment every time a small patch appears. If cyano repeatedly returns after chemical clearance, the tank is telling you the underlying cause remains.

What not to do

Do not drive nitrate and phosphate to zero

Coral and competing microorganisms need nutrients. Ultra-clean test results do not guarantee a healthy reef.

Do not add more flow without removing waste

Flow can spread detritus around the tank if filtration cannot capture it.

Do not deep-clean everything at once

A full sand wash, rock scrub, filter replacement and chemical treatment in one day can remove the stable biology the tank needs.

Do not assume a clean-up crew will solve it

Few common snails or hermit crabs consume enough cyanobacteria to control a bloom. Use them for general waste management, not as a red-slime cure.

Do not diagnose from colour alone

Dino and cyano can both appear brown, red and bubbly. Texture and behaviour matter.

Practical decision table

Observation	Recommended action
Small sheet in one sheltered corner	Siphon, clean the area and redirect flow
Cyano returns to the same sand patch	Check detritus, flow pattern and nutrient stability
Nitrate is zero but phosphate is measurable	Reduce aggressive nitrate removal and restore balance gradually
Both nutrients are elevated	Reduce input and improve export without a sudden crash
Mat covers coral	Remove it gently now and correct the surrounding conditions
Chemical treatment cleared it but it returned	Stop repeating the bottle and diagnose the root cause
Growth is stringy and disappears at night	Reassess for dinoflagellates
New tank has a light patch	Maintain stability and avoid extreme intervention

Related marine guides

• Dinoflagellates in Reef Tanks explains how dino differs and why UV is not a cyano cure.
• How to Cycle a Marine Aquarium in Malaysia covers biological maturation.
• Marine Aquarium Setup Malaysia covers source water, filtration and circulation planning.
• Cooling Fan, Evaporation and ATO helps prevent salinity instability while changing flow and aeration.

FAQ

Is cyanobacteria really algae?

No. It is photosynthetic bacteria, although reef keepers commonly call it red slime algae.

Does cyanobacteria always mean nutrients are too high?

No. It can occur in high-nutrient tanks, low-nitrate systems and tanks with unstable nutrient ratios. Detritus, flow and microbial disruption also matter.

Can I remove cyano by increasing flow?

Better circulation helps prevent settlement, but it will not remove the existing mat or correct overfeeding, dirty filtration or nutrient imbalance by itself.

Will a blackout cure cyanobacteria?

A blackout may reduce the visible mat temporarily. It does not fix the cause and can also stress photosynthetic coral if used repeatedly or for too long.

Should I use a cyanobacteria treatment?

Use one only after confirming the problem, removing biomass and correcting basic husbandry. Follow the manufacturer’s current instructions and protect oxygenation.

Can cyanobacteria kill coral?

Heavy mats can smother and irritate coral, block light and trap detritus. Remove growth that is contacting live tissue.

Why does cyano return after a water change?

The underlying flow, waste, source-water or nutrient pattern may still be present. A water change alone is not a complete control plan.

Do snails eat cyanobacteria?

Some animals may graze around it, but common clean-up crews are not reliable control for a significant bloom.

How long should control take?

Small outbreaks can improve quickly once the cause is corrected. Recurrent blooms may take weeks because the system needs stable conditions, not only visible removal.

Final advice

Cyano is easy to kill and surprisingly easy to bring back.

Siphon the mat, remove the waste feeding it, restore useful flow and keep nutrients stable. Use chemicals only when you have already fixed the conditions that caused the bloom.