What Is Columnaris?
Columnaris (Flavobacterium columnare) is one of the most misunderstood aquarium diseases — it's frequently misdiagnosed as a fungus because the white, cottony patches look fungal, but it's actually a gram-negative bacterial infection. This distinction is critical because antifungal medications don't work against columnaris, and using the wrong treatment wastes precious time while the disease progresses.
Columnaris exists in two forms:
- Chronic (slow): Develops over days to weeks. White patches spread gradually, fins erode, mouth may develop sores. Treatable if caught early.
- Acute (fast): Can kill fish within 24–72 hours. Often presents as sudden gill failure — the fish appears normal one day and is dead the next. By the time external symptoms appear, internal damage may be fatal.
Columnaris thrives in warm water (above 77°F) — unlike ich treatment where you raise temperature, with columnaris you should LOWER the temperature to 72–75°F to slow bacterial growth.
Treatment
- Lower temperature to 72–75°F gradually. Columnaris bacteria are more active in warm water — cooling slows their reproduction.
- Medicate with antibiotics:
- Seachem Kanaplex + Furan-2 combo: The most recommended treatment protocol in the hobby. Kanaplex (kanamycin) and Furan-2 (nitrofurazone) together cover a broad antibacterial spectrum. Dose both simultaneously per their respective instructions.
- API E.M. Erythromycin: Effective for milder cases.
- Aquarium salt: 1 tablespoon per 3 gallons can help — Flavobacterium doesn't tolerate salt well.
- Improve water quality immediately: 50% water change, clean filter, test with test kits.
- Remove activated carbon from filter during treatment.
- Treat the entire tank — columnaris is highly contagious among stressed fish.
Columnaris vs True Fungus — How to Tell
- Columnaris (bacterial): White/gray patches that may look cottony. Often starts at the mouth, edges of fins, or appears as a saddle-shaped lesion on the back. Progresses rapidly. More common in warm water.
- True fungus (Saprolegnia): Fluffy white growths that look like cotton balls. Usually appears on wounds or dead tissue. Grows more slowly. More common in cool water.
- Key distinction: If the white patches spread rapidly and the fish deteriorates quickly, suspect columnaris. If growth is slow and localized to a wound, suspect fungus.
- When in doubt: Treat with Kanaplex + Furan-2 (covers both bacterial and has some antifungal action).
Frequently Asked Questions
Why is columnaris called mouth fungus if it's bacterial?
The cottony white appearance at the mouth mimics fungal growth. The name "mouth fungus" predates the identification of the bacterial cause (Flavobacterium columnare). Despite the misleading name, antifungal medications won't work — antibacterials are required.
How fast does columnaris kill?
The acute form can kill within 24–72 hours. The chronic form progresses over days to weeks. Speed of treatment is critical — if you suspect columnaris, begin treatment immediately rather than waiting to see if it improves.