Columnaris vs Fungus in Fish: How to Tell the Difference
Why this matters
Many hobbyists see white, cottony patches and assume fungus. In reality, Flavobacterium columnare (columnaris) is a bacterium that often looks similar. Treating columnaris with an antifungal wastes valuable time.
Side-by-side comparison
| Feature | Columnaris (bacterial) | True fungus |
|---|---|---|
| Appearance | Pale, grayish or yellow patches, "saddle" on the back, frayed fins, mouth erosion | Fluffy, cotton-wool tufts that float in the water |
| Speed | Very fast, can kill within days | Slower, often secondary |
| Where it starts | Mouth, fins, gills, back | Damaged tissue, wounds, dead eggs |
| Warm water | Spreads faster in warm tanks | Less temperature dependent |
| Treatment class | Antibiotic active against gram-negative bacteria | Antifungal |
Treating columnaris
Columnaris is gram-negative, so aquarists commonly turn to antibiotics with gram-negative strength, such as a fluoroquinolone, in a hospital tank. Speed matters because it can progress rapidly. Lowering stress and keeping water clean also helps.
Treating true fungus
Genuine fungal infections are less common. When confirmed, azole antifungals such as ketoconazole or fluconazole are options. Fungus often follows injury or poor conditions, so correcting those is part of the cure.
Frequently asked questions
What does columnaris look like?
Typically pale or grayish patches, frayed fins, and a saddle-shaped mark on the back. Mouth lesions can look like cotton, which is why it is often mistaken for fungus.
Is cotton-like growth always fungus?
No. Mouth and body lesions caused by columnaris often look cotton-like but are bacterial. True fungus is fluffy and tends to appear on damaged tissue.
How fast does columnaris kill fish?
In warm water it can kill within a few days, which is why early treatment matters.