Cyanobacteria (Red Slime) in a Reef Tank

A cyanobacteria bloom is one of the most common nuisance problems in reef-keeping, and one of the most visually alarming — a red or purple film that can carpet the sandbed and rockwork within days. It looks dramatic, but it is also one of the more mechanically understandable problems: cyano thrives on a specific, identifiable combination of conditions, and correcting those conditions resolves most outbreaks without resorting to antibiotics or a full tank teardown.

What cyanobacteria actually is

Despite the common nickname "red slime algae," cyanobacteria is not a true algae — it is a photosynthetic bacteria, sometimes called blue-green algae for historical reasons. It exists at low background levels in essentially every reef tank, living in the biofilm on rock and sand. Under favourable conditions it multiplies rapidly and forms a visible sheet, most often a deep red or maroon colour, though green and even blackish forms occur too.

How to identify cyanobacteria

Correct identification matters because the nutrient-management response for cyano is the opposite of the response to a low-nutrient dinoflagellate bloom, and the two are sometimes confused at a glance.

• Colour: most often deep red or purple; green and dark forms also occur
• Texture: a smooth, continuous sheet or mat rather than individual strands or a tuft
• Removal: peels off in one film when siphoned or brushed, without leaving fibres behind
• Bubbles: can trap small gas bubbles under the sheet during the light period, but does not lift and pearl the way dinoflagellate strands do
• Location: settles first in low-flow dead spots — behind rockwork, in sandbed corners, along the substrate edge

Why cyanobacteria blooms happen

Two conditions drive most outbreaks, usually together. The first is elevated dissolved organics and nutrients — overfeeding, a heavy bioload, infrequent water changes, or a filter sock or media that has gone too long without cleaning all feed the bacteria a steady food source. The second is weak or uneven flow. Cyanobacteria colonises dead spots where detritus settles and oxygen exchange is poor; a tank with strong, well-distributed flow gives it far less real estate to establish on.

New tanks are also prone to an early cyano phase during the first few months as the system's bacterial and microalgae community is still establishing — this is usually self-limiting and tends to fade as the tank matures, provided flow and nutrient export are reasonable.

The recovery approach

Cyanobacteria recovery is faster than a dinoflagellate outbreak in most cases — typically one to three weeks of steady improvement — because the fix works with the tank's existing nutrient-export mechanisms rather than against them.

1. Improve flow first. Reposition or add powerheads to eliminate dead spots, particularly low-flow corners and behind rockwork. This alone often visibly slows a bloom within days.
2. Increase nutrient export. Step up water change frequency temporarily, check that the skimmer is pulling a wet, dark skimmate, and clean or replace filter socks and mechanical media more often while the bloom is active.
3. Reduce the organic input. Cut back on overfeeding and remove uneaten food promptly. This is not the same as starving the tank the way a dino recovery does — the goal is trimming excess, not driving nutrients to zero.
4. Manually siphon daily. Cyano sheets lift off cleanly with a turkey baster or siphon. Removing the visible mat each day, before it spreads further, reduces the reproducing surface area while the underlying causes are corrected.
5. Reserve chemical treatments for stubborn cases. Products designed to knock back cyanobacteria exist and can help a severe bloom, but they treat the symptom, not the cause — flow and nutrient export still need addressing, or the bloom typically returns once dosing stops.

Tracking your way through recovery

Cyano recovery is easy to misjudge by eye alone — the bloom can look the same from one day to the next even as the underlying nutrients are trending down. Logging nitrate and phosphate through the recovery period, alongside a quick note each time you adjust flow or increase water changes, turns a vague sense of "it seems a bit better" into a visible trend. Recording your own readings and observations this way is what lets you tell whether the changes you made are actually working, or whether it is time to adjust the approach before the bloom gets worse. That is the whole value of keeping a proper log next to the tank rather than relying on memory.