fish

Columnaris (Cotton Mouth)

A bacterial infection (Flavobacterium columnare) causing white/gray patches on the mouth, fins, or body. Can progress rapidly and become fatal without treatment.

Symptoms

White or gray cotton-like patches on the mouth, fins, or body, often starting at the mouth (cotton mouth).
Frayed or eroded fins, pale gills, and a slimy film over the skin.
Rapid onset — lethargy, loss of appetite, and possible death within a day or two in severe cases.

Common Causes

Infection by the bacterium Flavobacterium columnare, which multiplies when fish are stressed.
Stress from transport, crowding, temperature swings, or handling that lowers immunity.
Poor water quality (ammonia, nitrite, low oxygen) that weakens the fish’s protective slime coat.

What to Check

Water quality first — ammonia, nitrite, nitrate, temperature, and oxygen levels.
Inspect the mouth, gills, and fins for gray-white patches under good light.
Check recently added or handled fish — outbreaks usually start with stressed arrivals.

What Not to Do

Do not wait for the patches to spread — columnaris can become fatal within a day or two.
Do not treat without correcting water quality — medication alone usually fails in poor water.

This guide is informational and does not replace professional veterinary advice. If fish are severely ill, consult a veterinarian.

Columnaris (Cotton Mouth)

Symptoms

Common Causes

What to Check

What Not to Do

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Columnaris (Cotton Mouth)

Symptoms

Common Causes

What to Check

What Not to Do