# Columnaris vs Fungus in Fish: How to Tell the Difference

> Columnaris is a fast-spreading bacterial infection that causes pale, frayed or saddle-shaped patches and eroded fins, while true fungus forms fluffy, cotton-like growth, usually on dead or damaged tissue. Because they need different medications, an antibiotic for columnaris and an antifungal for fungus, identifying the right one comes first.

Source: https://petremedyusa.com/guides/columnaris-vs-fungus/ · Updated 2026-09-30 · PetRemedyUSA

## 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](https://petremedyusa.com/medications/ciprofloxacin-for-fish/), in a [hospital tank](https://petremedyusa.com/guides/hospital-tank-setup/). 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](https://petremedyusa.com/medications/ketoconazole-for-fish/) or [fluconazole](https://petremedyusa.com/medications/fluconazole-for-fish/) are options. Fungus often follows injury or poor conditions, so correcting those is part of the cure.

> **If unsure,** treat it as bacterial and act quickly, since columnaris is the more dangerous possibility, and consult an aquatic veterinarian. Our [Fungus and Columnaris Pack](https://petremedyusa.com/shop/treatment-packs/) covers both possibilities.

## 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.