Columnaris in Fish: It Looks Like Fungus, But It Is Not

The short version

What columnaris actually is

Columnaris is a bacterial infection caused by Flavobacterium columnare (older texts call it Flexibacter columnaris).¹ The bacteria live in most freshwater tanks at low levels and turn dangerous when a fish is stressed. The signature sign is a pale, cottony-looking patch, classically on the mouth, which is why hobbyists call it cotton mouth or, wrongly, mouth fungus.²

That nickname is the most dangerous thing about it. Columnaris is not a fungus. It just looks like one. As one fish vet puts it, it’s a bacteria that looks like a fungus, and anti-fungal treatments don’t work on it.¹ Treat it as fungus and you lose the narrow window that decides whether the fish lives.

Columnaris vs. fungus: the difference that saves fish

This is the single most useful thing to get right. Because columnaris and true fungus can look similar, both pale and fluffy, owners often reach for an antifungal, watch it do nothing, and lose days they couldn’t spare.² Here is how the two differ.

A few practical tells: true fungus tends to grow outward in cotton-like threads from an injury or dead tissue, while columnaris looks flatter and slimier and often starts on the mouth or as a pale “saddleback” patch across the back.² Columnaris also spreads noticeably faster. The only certain way to tell them apart is a microscope check, which a fish vet can do quickly, and when in doubt, because columnaris moves so fast, it’s safer to treat for the bacterial infection.¹

Signs of columnaris

Columnaris can show in several places, sometimes at once. Watch for:

Gill involvement is especially urgent: when columnaris attacks the gills, a fish can decline very quickly, sometimes before obvious external patches appear.

What triggers columnaris

Because the bacteria are already present in most tanks, columnaris is almost always a stress disease. It flares when something lowers a fish’s defenses: warm water (the bacteria thrive as temperatures rise), poor water quality, crowding, recent shipping or handling, injury, and low oxygen.³ Newly bought or recently moved fish are classic cases, which is one more reason to quarantine new arrivals.

The warm-water link matters for bettas especially. Bettas are kept warm, and warm tanks let columnaris move faster. So a betta with a pale, fuzzy mouth needs a quick, correct response, not an antifungal and a wait.

How to treat columnaris

Speed and the right target are everything. The plan below is the clear version; dosing and drug choice are a fish vet’s call.

1. Act fast and lower the stress

Move affected fish to a hospital tank if you can, and improve water quality right away with testing and partial water changes.⁴ Because columnaris speeds up in warm water, gradually lowering the temperature can slow it, check what’s safe for your species first. Aquarium salt helps some species by supporting gill function and stressing the bacteria, but it’s not safe for every fish, so confirm before using it.³

2. Use the right kind of medicine, antibacterial, not antifungal

This is where the fungus mix-up costs fish. Columnaris needs an antibacterial. Flavobacterium is gram-negative, so a vet will choose a drug that covers gram-negative bacteria; some older or milder products are known to work poorly against it.¹ Remove activated carbon before dosing, since it strips medication from the water. Which drug, and at what dose, is a decision for a fish vet, ideally guided by what the infection looks like and, where possible, testing.

If a vet decides medicine is needed

Preventing columnaris

Prevention is stress control. Keep the tank clean, cycled, and not overcrowded, and avoid keeping fish warmer than they need to be. Quarantine new fish before adding them, since shipping stress and unfamiliar bacteria are a classic trigger. Handle fish gently to avoid the small injuries columnaris exploits, feed a good diet, and clean nets and siphons between tanks so you don’t carry the bacteria from a sick system to a healthy one. A calm, clean, well-run tank rarely sees it.

Frequently asked questions about columnaris

Related guides

• Fish popeye, a related condition with a similar gram-negative bacterial cause.
• Fish dropsy, a serious internal disease that can follow an untreated infection; know the difference from fin rot.