Columnaris is a bacterial infection of the skin, fins, mouth and gills, caused by Flavobacterium columnare. It is often mistaken for fungus because of its white, fluffy look. It can move fast, so it pays to recognize it early.
What columnaris looks like
- Grey-white or yellowish patches on the body, often on the back. A band across the back is sometimes called “saddleback”.
- White, cotton-like growth around the mouth, often called “mouth fungus” even though it is bacterial.
- Frayed fins that look eaten away from the edges.
- Fast breathing or gasping when the gills are affected.
The Merck Veterinary Manual notes that columnaris is most common in warmwater fish, that losses can be sudden, and that the disease can be hard to control once it is noticed (Bacterial Diseases of Fish).
Columnaris or fungus?
| Columnaris (bacterial) | True fungus | |
|---|---|---|
| Look | Flat or slightly raised grey-white patches, can look slimy | Fluffy, cotton-wool tufts |
| Where | Back, mouth, fin edges, gills | Usually on wounds, damaged tissue or dead eggs |
| Speed | Can spread within a day or two | Usually slower |
| Treated with | A product labeled for bacterial infections such as columnaris | An antifungal |
An aquatic veterinarian can confirm columnaris by looking at a skin or gill sample under a microscope.
What triggers it
Outbreaks usually follow stress: poor water quality, crowding, rough handling, shipping or a sudden temperature change. Newly bought fish are at higher risk, which is why quarantine matters.
What to do
- Test the water and fix ammonia, nitrite or temperature problems with partial water changes.
- Move affected fish to a hospital tank. Columnaris spreads between fish, so act early and watch the others closely.
- Use a product labeled for columnaris, such as Fish Zithro (fish azithromycin), and follow the label’s dose and schedule.
- Keep the temperature steady and don’t raise it, since columnaris is most common in warm water.
- Disinfect nets and equipment used on sick fish before using them in other tanks.
When to call a veterinarian
Ask an aquatic veterinarian if several fish are sick, if fish are dying, or if signs keep spreading after a day or two of treatment. A vet can culture the bacteria and test which drugs work; Merck recommends this sensitivity testing before antimicrobial use.
Preventing columnaris
- Quarantine new fish for at least 30 days.
- Keep ammonia and nitrite at zero with regular partial water changes.
- Avoid overcrowding, and handle and net fish gently.
Browse all fish antibiotics, compared by ingredient, strength and count.
Fish antibiotics are for ornamental aquarium fish only: not for human use and not for food-producing animals. This guide is general information, not veterinary advice; for valuable or persistently sick fish, ask an aquatic veterinarian. Last reviewed: November 2026.