What is red slime algae?

It is not really an algae - it is cyanobacteria, a photosynthetic bacteria. It shows up as a slimy red, maroon, green or black mat or sheet on sand, rock and glass, usually with tiny oxygen bubbles trapped in it. Those trapped bubbles are the giveaway.

Why do I have cyanobacteria if my nitrate and phosphate are low?

Cyano can bloom even when your tests read low or zero - it is opportunistic and feeds on dissolved organics and detritus. It is more about nutrient IMBALANCE and dead spots than a single high number, which is why bottoming your nutrients to zero does not fix it (and can make things worse).

How do I get rid of red slime naturally?

Siphon it out (repeatedly, during water changes), increase flow to kill the low-flow dead spots it loves, improve nutrient export with good skimming and sensible feeding, and keep nutrients low but not zero. Fix the flow and nutrient balance and it recedes; that is the lasting cure.

Do red slime remover chemicals work?

They work fast, but they are essentially an antibiotic (erythromycin) that treats the symptom, not the cause - so cyano returns if you have not fixed the flow and nutrients. They also lower oxygen and disrupt your biofilter, so you must run an airstone and turn the skimmer off during treatment. Use as a last resort, not a first move.

Is cyanobacteria dangerous to my tank?

A film is mostly a nuisance and an eyesore, but a heavy mat can smother corals and coralline, and it signals that flow or nutrient handling is off. Treat it as a warning light: fix the underlying cause and it clears.

Cyanobacteria (Red Slime): How to Fix It

🐠 The short version: red slime is cyanobacteria — a photosynthetic bacteria, not an algae — and the trapped oxygen bubbles are the tell. It blooms in low-flow dead spots with a nutrient imbalance (it can even appear when tests read zero). The lasting fix is husbandry: siphon it out, add flow, improve export, and keep nutrients low but not zero. Chemical “removers” are an antibiotic that treats the symptom, not the cause.

🔧 Not sure if your nutrients are actually the problem? Check them against the parameter target chart and log the trend with the parameter tracker.

What it actually is

Cyanobacteria — “cyano,” or red slime — isn’t a true algae; it’s a photosynthetic bacteria. It appears as a slimy sheet or mat, usually deep red or maroon (sometimes green or black), draped over sand, rock and glass. The diagnostic sign is the little oxygen bubbles trapped in the film — a byproduct of its photosynthesis. A patch you can peel off in a sheet is almost always cyano.

Why it blooms (it’s not just “too much nitrate”)

This is the part that trips people up. Cyano is opportunistic and feeds on dissolved organics and detritus, so it can bloom even when your nitrate and phosphate test at or near zero. The real drivers are:

Low or stagnant flow — it colonises the quiet dead spots where detritus settles.
A nutrient imbalance — overfeeding and low-quality foods load the water with organics; and bottoming your nutrients out to true zero actually invites problems rather than solving them.
Warm water and low alkalinity are contributing conditions.

So “just lower your nutrients to zero” is the wrong instinct — the target is low but not zero, with good flow and export.

The fix that lasts

Work the cause, not just the symptom:

Siphon it out — physically remove the mats during water changes, as often as it takes (even daily in a bad bloom).
Add flow — extra powerheads or repositioned ones to eliminate the dead spots it loves. This is often the single biggest lever.
Improve nutrient export — keep the skimmer running well, feed sensibly, and do regular water changes.
Balance, don’t starve — keep nitrate and phosphate low but measurable; if they’re bottomed out, feed a little more.

Fix flow and nutrient balance and cyano recedes on its own — that’s the durable cure.

The chemical option (and its honest catch)

“Red slime removers” are essentially erythromycin, a broad-spectrum antibiotic. It clears cyano fast, but the catches are real:

It treats the symptom, not the cause — cyano comes back if the flow/nutrient problem isn’t fixed.
As an antibiotic it can disrupt your biofilter and drop oxygen levels, so you must run an airstone and turn the skimmer off during treatment, then replace carbon afterward.

Use it as a last resort to knock back a severe bloom while you fix the underlying husbandry — never as the first move.

The honest takeaway

Red slime is a message, not a mystery: somewhere you’ve got a dead spot, too much organic load, or nutrients out of balance. Siphon, add flow, export better, and hold nutrients low-but-not-zero, and it fades. Reach for the antibiotic only to buy time while you fix the real cause.

Sources & further reading

Coralkin — Beating reef tank algae & pests and water chemistry explained — the husbandry context.
Coralkin — Should nitrate & phosphate be zero? — why low-but-not-zero matters here.
Bulk Reef Supply — “Red Slime: How to Prevent and Remove Cyanobacteria”, bulkreefsupply.com — bacteria not algae, trapped bubbles, siphon + flow + export, the antibiotic caveat.
Reef Builders — “What is Red Slime and How to Avoid It”, reefbuilders.com — dead-spot/flow link; chemicals return if the cause isn’t fixed.