Columnaris in Betta Fish: The 'Cotton' Disease That Isn't Fungus

What Columnaris Is (and Why It Isn't Fungus)

If your betta suddenly grows a patch of white or grey cottony fuzz on its mouth, back, or fins, the instinct is to reach for an antifungal. Don't. That mold-like growth is one of the aquarium hobby's great impostors: columnaris, a bacterial disease that only looks like fungus. It is caused by bacteria called Flavobacterium columnare [1]. The flagship extension fact sheet on it is even titled a "bacterial infection caused by Flavobacterium columnare" [2] — a name worth remembering, because it tells you the single most important thing about treatment. Antifungal medicines have nothing here to attack; they will not cure a bacterial infection, and the days you lose trying one can cost your fish its life.

Columnaris hides behind several folk names that add to the confusion — "cotton mouth," "cotton wool disease," "mouth fungus," and "saddleback disease." Veterinarians know it as saddleback or cottonmouth disease [1]. Whatever you call it, the organism is a bacterium, and it is common in warm-water fish like bettas [1].

Signs of Columnaris

Columnaris can appear almost anywhere on the body, which is part of what makes it tricky to spot. Watch for:

• White, grey, or pale-yellow cottony patches, or a filmy, slimy coating, on the mouth, fins, gills, or body. Merck describes the lesions as having a "slimy or cotton-like" surface [1].
• A pale band or "saddle" across the back near the dorsal fin — the classic saddleback lesion. In documented cases the discoloration and ulceration cluster on the dorsal fin and trunk, as well as the mouth, lips, and gill arches [4].
• Frayed or eroding fins, and visible erosion around the mouth.
• Rapid gill movement or labored breathing, especially when the gills are involved.
• Early behavioral clues: loss of appetite, lethargy, and hanging near the water surface [4].

Because the infection can move fast, treat any spreading cottony patch as urgent rather than waiting to "see how it goes."

Why Warm Water Makes It Worse

Here is the counter-intuitive part. With many fish problems — ich, for example — keepers are told to nudge the heater up. With columnaris, do the opposite of your instinct: do not raise the temperature. Heat feeds this bacterium. A veterinary review found that mortality from F. columnare rose with temperature, with higher losses at 23°C than at 18°C, and that transmission between fish is more efficient in warmer water [3]. That same review reported that highly virulent strains killed young salmon within 12 to 24 hours at 20°C in laboratory studies [3].

Bettas are tropical and do need a stable, heated tank in their normal range (roughly 78–80°F). The point is not to chill them — it is to avoid cranking the heater higher in the mistaken belief that heat fights infection. Keep the temperature steady and in the normal betta range, and double-check that a faulty heater isn't quietly running the tank hot.

Why Bettas Get Columnaris

Flavobacterium columnare is an opportunist. It is often already present in the environment and turns dangerous when a fish is stressed or the water goes bad. The veterinary review links outbreaks to high organic load and nitrite, high stocking density, and physical injury or abrasion that lets bacteria enter the tissue [3]. For a betta, that maps onto familiar culprits: ammonia or nitrite spikes from an uncycled or dirty tank, overcrowding, rough handling or torn fins, and — as above — water that is too warm. Poor water quality plus warmth is the perfect storm.

Columnaris vs. Fin Rot, Fungus, and Popeye

Getting the diagnosis right decides which medicine you buy:

• True fungal infection: Real fungus (usually Saprolegnia) also looks cottony, but it tends to form more discrete, fluffy white tufts on wounds or dead tissue and does respond to antifungals. Columnaris looks similar but is bacterial [1] — so the wrong medicine wastes critical time.
• Fin rot: Fin rot eats away at fin edges, leaving ragged, receding margins. Columnaris can fray fins too, but it also strikes the mouth, gills, and back with that cottony coating.
• Popeye: Popeye is swelling or bulging of the eye, not a surface fuzz — a different problem needing a different approach.

Managing Columnaris

Move quickly, and roughly in this order:

1. Fix the water first. Test for ammonia and nitrite and do a partial water change to bring them down. Reducing organic load and avoiding injuries are central to controlling columnaris [6].
2. Check the temperature. Confirm the tank isn't running hot, and keep it steady in the normal betta range rather than raising it [3].
3. Quarantine. Because it spreads more readily in warm water [3], isolate an affected betta — and any new arrivals — in a separate tank to protect tankmates.
4. Consider aquarium salt cautiously. Some keepers add aquarium salt as a general, stress-reducing support. The evidence for salt against columnaris is mixed and species-dependent, so treat it as a supplement — never a replacement for proper medication.
5. Use an antibacterial medication for anything beyond the mildest case. Caught very early, potassium permanganate or hydrogen peroxide can help; once the infection turns chronic or systemic, antibiotics such as florfenicol or oxytetracycline are recommended [6]. Severe, fast-moving cases usually need a real antibacterial, and a fish or aquatic veterinarian can prescribe and guide treatment. By the time columnaris is obvious, it can already be difficult to control [5].

Preventing Columnaris

Prevention is mostly good keeping: a fully cycled tank with stable, clean water and zero ammonia and nitrite; a steady temperature in the correct range (not too warm); low stress from gentle handling and no overcrowding; and a quarantine period for every new fish or plant before it joins your display tank. A calm, clean, appropriately warm tank gives this opportunist very little to work with.

When to See a Vet

Columnaris can be an emergency. Consult an aquatic or fish veterinarian, or act urgently, if:

• A cottony or pale patch is spreading visibly within hours, or your betta's mouth or gills are affected — gill infection can be rapidly fatal.
• Your betta stops eating, turns very lethargic, or breathes rapidly at the surface.
• More than one fish in the tank develops lesions, signaling an active outbreak.
• The fish doesn't improve within a day or two of correcting the water and starting an appropriate antibacterial.

Frequently Asked Questions

Is columnaris a fungus?

No. Despite the cottony, fuzzy look, columnaris is a bacterial infection caused by Flavobacterium columnare [1]. That is exactly why antifungal treatments don't work on it — you need an antibacterial approach instead.

Should I raise the tank temperature to treat columnaris?

No — this is the opposite of the right move. A veterinary review found that columnaris mortality rose with temperature and that the bacterium spreads more efficiently in warmer water [3]. Keep your betta's tank steady in its normal range and make sure the heater isn't accidentally running hot.

How fast can columnaris kill a betta?

It can be very fast. In laboratory studies, highly virulent strains of F. columnare killed young salmon within 12 to 24 hours [3]. Because it moves so quickly, treat any spreading cottony patch as urgent.

What medication treats columnaris?

Antibacterials, not antifungals. Caught early, potassium permanganate or hydrogen peroxide may help; chronic or systemic infections are treated with antibiotics such as florfenicol or oxytetracycline [6]. A fish vet can help you choose and dose the right one.

Is columnaris contagious to my other fish?

Yes. It transmits between fish more readily in warm water [3], so an affected betta in a community tank can pass it to tankmates. Quarantine both affected and newly bought fish.

How is columnaris different from a true fungal infection?

Both can look like white cotton, but columnaris is bacterial while a true fungal infection (often Saprolegnia) is a genuine fungus [1]. They need different medicines, so getting it right matters — antifungals won't cure columnaris.

References

1. Merck Veterinary Manual. Disorders and Diseases of Fish. https://www.merckvetmanual.com/all-other-pets/fish/disorders-and-diseases-of-fish
2. Durborow RM, Thune RL, Hawke JP, Camus AC. Columnaris Disease: A Bacterial Infection Caused by Flavobacterium columnare (SRAC Publication No. 479), UF/IFAS. https://aquaculture.mgcafe.uky.edu/sites/aquaculture.ca.uky.edu/files/srac_479_columnaris_disease_-_a_bacterial_infection_caused_by_flavobacteriun_columnare.pdf
3. Declercq AM, Haesebrouck F, Van den Broeck W, Bossier P, Decostere A. Columnaris disease in fish: a review with emphasis on bacterium–host interactions. Veterinary Research. https://pmc.ncbi.nlm.nih.gov/articles/PMC3648355/
4. University of Guelph, Animal Health Laboratory. Columnaris disease. https://www.uoguelph.ca/ahl/columnaris-disease-rainbow-trout
5. Merck Veterinary Manual. Bacterial Diseases of Fish (Aquarium Fish). https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquarium-fish/bacterial-diseases-of-fish
6. Merck Veterinary Manual. Bacterial Diseases in Aquaculture. https://www.merckvetmanual.com/exotic-and-laboratory-animals/aquaculture/bacterial-diseases-in-aquaculture