Fish & shrimp health guide
Columnaris (Cotton Mouth)
Also called mouth fungus, cotton mouth, Flavobacterium
White or grey cottony patches around the mouth and body, and greyish saddle marks, spreading fast.
TL;DR, Columnaris (Cotton Mouth)
Columnaris is a bacterial infection that masquerades as fungus, which is why it is often called mouth fungus. It shows as white or grey fuzzy patches around the mouth, greyish saddle-shaped marks on the back, and frayed fins, and it spreads alarmingly fast, especially in warm water. Because it looks fungal but is bacterial, antifungal treatments fail and waste time you do not have. Treat it as the fast bacterial infection it is, and lower the temperature rather than raising it.
What you see
White to grey cottony growths on the mouth, a greyish saddle mark across the back, ragged fins, and skin lesions. Fish go off food and lethargic. It can kill within a day or two in warm tanks, so speed matters.
What causes it
- High temperature, which accelerates the bacteria
- Poor water quality and high organic waste
- Stress, injury, or a new fish bringing it in
- Crowding that spreads it quickly between fish
How to treat it
- Lower the temperature, the opposite of ich, since warmth makes columnaris more aggressive.
- Use an antibacterial or antibiotic treatment effective against columnaris, not an antifungal.
- Improve water quality immediately with a change and clean maintenance.
- Isolate affected fish if you can, because it spreads fast.
- Act within hours, not days, in a warm tank.
Treatment safety in a planted tank
The antibiotics used for columnaris can dent your biofilter and stress inverts. Treat in quarantine when possible and watch ammonia in the display if you dose there. Do not reach for antifungal remedies, they do nothing to this bacterium.
Prevent it
Keep water clean, avoid overcrowding, quarantine new fish, and don't run the tank hotter than the fish need. Columnaris preys on stressed fish in warm, dirty water.
Easy to confuse with
True fungus (Saprolegnia) is a genuine cottony growth but grows slowly and usually on injuries or dead tissue. Columnaris spreads fast and hits healthy-looking fish, and it needs antibacterial, not antifungal, treatment.
Species most at risk
Species that catch this most often, or that need a gentler treatment:
Frequently asked questions
Direct answers to the questions search engines and AI assistants surface most often about this species.
Is it fungus or bacteria?
Bacteria, despite the cottony look and the mouth-fungus name. Antifungal treatments will not work, use an antibacterial.
Should I raise the temperature like for ich?
No, do the opposite. Heat makes columnaris far more aggressive, so lower the temperature.
Why is my fish dying so fast?
Columnaris is one of the fastest killers in the hobby, especially in warm water. Treat within hours of spotting it.