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Columnaris (Cotton Mouth Disease) in Fish
A highly contagious bacterial infection caused by Flavobacterium columnare. Often confused with fungal infections due to its cotton-like appearance. Can kill fish within 24-48 hours in acute cases.
Last updated: 2026-05-08
Severity
moderate
When to Act
See Vet Today
Symptoms & Signs
Cotton-like growths
White, gray, or yellowish cottony patches on the mouth, fins, or body — often with a reddish ulcer underneath.
Eroded mouth and fins
"Cotton mouth" — the mouth area appears eaten away with white fuzz. Fin edges become ragged.
Rapid gill movement
Labored breathing if gills are affected.
Lethargy
Fish stops swimming actively and may rest on the bottom.
Behavioral Changes to Watch For
Pets can't tell us what's wrong. These behavioral changes are often the first clues that something is wrong.
🐾 Clamped fins and hiding
Fish holds fins close to the body and hides.
What You May Notice:
Your fish that normally begs for food is hiding behind plants with fins tight against its body.
Causes & Risk Factors
Causes
- •Infection with Flavobacterium columnare — a gram-negative bacterium
- •The bacteria is ubiquitous in freshwater aquariums — it causes disease when fish are stressed
- •Enters through damaged skin or gills
- •Thrives in warm, organically rich water
Risk Factors
- ⚠Poor water quality (high organic load, high ammonia/nitrite)
- ⚠Overcrowding
- ⚠Recent transport or handling stress
- ⚠High water temperature (bacteria thrives above 76°F/24°C)
- ⚠Physical injury from netting, fighting, or rough decorations
How It's Diagnosed
- 1Visual identification — cottony patches with underlying ulceration distinguish it from true fungus
- 2Microscopic examination of a scraping — columnaris bacteria form "haystack" formations
- 3Gram stain — gram-negative filamentous rods
Treatment Options
Antibiotic Treatment
Columnaris is bacterial, not fungal — antifungal medications will NOT work.
Steps
- 1.Kanamycin (Kanaplex) + nitrofurazone (Furan-2) — most effective combination
- 2.OR trimethoprim-sulfa in food if fish is still eating
- 3.Treat for 5-7 days
- 4.Remove activated carbon during treatment
- 5.Lower tank temperature to 72-74°F (22-23°C) if species-appropriate — slows bacterial growth
Expected Outcome
Lesions stop spreading within 2-3 days. Complete resolution in 7-10 days.
Precautions
- !Many "fungus treatments" are useless against columnaris — it's a bacterium, not a fungus
- !Antibiotics will impact the biological filter — monitor ammonia/nitrite during treatment
Water Quality and Environmental Correction
Underlying husbandry issues must be fixed or the infection will return.
Steps
- 1.Immediate 50% water change with dechlorinated water
- 2.Gravel vacuum to remove organic debris
- 3.Reduce feeding to once daily during treatment
- 4.Improve overall tank hygiene
Expected Outcome
Reduced bacterial load in the water and reduced stress on the fish.
Precautions
- !Don't over-clean — preserve beneficial bacteria in the filter
Common Medications Used
| Medication | Usage | Important Notes |
|---|---|---|
| Kanamycin (Kanaplex) | Broad-spectrum antibiotic effective against gram-negative columnaris | Absorbed through skin and gills. Excellent for external/internal bacterial infections. |
Prevention
- ✓Maintain excellent water quality — weekly water changes
- ✓Do not overstock the aquarium
- ✓Quarantine all new fish for 4 weeks
- ✓Minimize handling and netting stress
- ✓Prompt removal of uneaten food and dead plant matter
When to See a Veterinarian
- ⚠️Cotton-like patches on the mouth or body
- ⚠️Rapidly spreading white/gray lesions
- ⚠️Multiple fish affected simultaneously
- ⚠️Fish gasping or breathing rapidly
Frequently Asked Questions
Is it fungus or columnaris?
Prognosis
Good if treated early before the infection becomes systemic. The acute form can kill within 24-48 hours without treatment. Chronic cases may smolder for weeks with waxing and waning lesions.
References
- [1] Noga — Fish Disease
- [2] AVMA — Aquatic Medicine
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moderate