Betta columnaris — also called cotton mouth or mouth fungus — is one of the fastest-killing infections in the hobby and the single most commonly misdiagnosed, because it looks exactly like a fungus but is actually a bacterial disease that antifungal medication cannot touch. This guide gives you a fast, accurate way to identify columnaris versus true fungus, the exact gram-negative antibiotic and cool-water protocol that beats it, a 24-hour emergency action plan, and the husbandry that prevents outbreaks. With betta columnaris, recognising it correctly within hours is the difference between life and death — so read the identification section first.
Why Columnaris Is So Dangerous
Columnaris is caused by the bacterium Flavobacterium columnare. People call it “mouth fungus” because it looks cottony, but treating it with antifungal medication wastes time — and with columnaris, time is everything. Some fish die within 24–48 hours of symptoms appearing. Speed and correct identification are the whole game.
Columnaris vs. True Fungus (critical)
| Columnaris (bacterial) | True fungus (Saprolegnia) | |
|---|---|---|
| Look | Flat, filmy, grayish-white, slightly yellow/brown tint; mouth/saddle | 3-D fluffy cotton tufts, pure white |
| Location | Mouth, gills, a “saddle” patch on the back | Usually on a wound or dead tissue |
| Speed | Very fast — can kill in 24–48h | Slower |
| Treatment | Antibiotic (gram-negative) | Antifungal |
Rule of thumb: if there is any doubt whether it’s fungus or columnaris, treat for columnaris — it’s the one that kills fast. Compare with our betta fungal infection guide to see the visual difference.
Symptoms
- Grayish-white, fuzzy or slimy patches on the mouth (cotton mouth), body, or a “saddleback” patch.
- Frayed/ragged fins, mouth erosion in later stages.
- Gills turning brown, rapid breathing if gills are hit.
- Lethargy, loss of appetite, clamped fins.
Step-by-Step Betta Columnaris Treatment
Step 1 — Isolate immediately
Move the betta to a hospital tank — columnaris is highly contagious and fast. See how to quarantine a new betta for a quick hospital setup.
Step 2 — Lower the temperature
Columnaris thrives in warm water. Slowly lower to 75–77°F (the low end of the betta comfort range) to slow the bacteria. This is the opposite of ich/velvet — do not heat.
Step 3 — Antibiotics (the actual cure)
Use a gram-negative antibiotic. A widely used combination is kanamycin + furan-2; alternatives include doxycycline or minocycline products. Remove filter carbon before dosing and complete the full course. Details in our betta medication guide.
Step 4 — Aquarium salt + clean water
Add aquarium salt (1 tsp/gallon) to reduce stress and support the immune system, and keep water spotless with daily changes.
What NOT To Do
- Don’t heat the tank (that’s for ich/velvet — it makes columnaris worse).
- Don’t treat it as fungus.
- Don’t wait “to see how it develops” — act within hours.
- Don’t keep the sick fish with tank mates.
Prevention
Columnaris outbreaks follow stress, injury, poor water, and un-quarantined newcomers. Stable clean water, gentle handling, quarantine, and low stress are the defence — see how to prevent betta diseases and signs of stress in betta fish.
The 24-Hour Columnaris Action Plan
Columnaris can kill in 24–48 hours, so there’s no time for a “let’s see how it develops” approach. If you strongly suspect it, run this timeline immediately:
| Time | Action |
|---|---|
| Hour 0 | Isolate the betta in a heated hospital container; remove filter carbon |
| Hour 0–1 | Begin lowering temperature toward 75–77°F (slowly); add aquarium salt (1 tsp/gal) |
| Hour 1–2 | Start the gram-negative antibiotic (e.g., kanamycin + furan-2) at label dose |
| Day 1–2 | Daily clean water; continue full antibiotic course; do NOT stop early |
| Ongoing | Disinfect nets/equipment; watch tank mates for spread |
The fatal mistake is treating it as fungus and losing those first 24 hours to an antifungal that does nothing. With columnaris, speed of correct action — not perfect certainty — saves the fish.
Why “Cotton Mouth” Fools Almost Everyone
Columnaris is bacterial, but its cottony, fuzzy appearance screams “fungus” to the eye — which is exactly why it’s so deadly. Cement these distinctions:
- Location: mouth, gills, or a “saddle” patch on the back = classic columnaris. Random tuft on a wound = more likely true fungus.
- Texture: columnaris is flat, filmy, grey-white with a yellow/brown tint and spreads fast; fungus is a discrete fluffy 3-D white ball that spreads slowly.
- Temperature direction: columnaris thrives in warm water — you cool the tank (opposite of ich/velvet). Getting this backwards accelerates it.
- The safe rule: if you genuinely can’t tell fungus from columnaris, treat for columnaris. It’s the one that kills in two days.
Outbreaks follow stress, injury, poor water, and un-quarantined newcomers — so the long game is prevention: stable water, gentle handling, and strict quarantine, exactly as in how to prevent betta diseases.
What Is Columnaris? A Clear Definition
Columnaris is a disease caused by the bacterium Flavobacterium columnare (formerly Flexibacter columnaris), a gram-negative rod-shaped bacterium that is present in virtually all freshwater systems. It becomes deadly when a fish is stressed, injured, or kept in poor or warm water. The bacterium attacks the skin, fins, mouth, and gills, producing the characteristic pale, filmy, “cottony” lesions that give it the misleading nickname “cotton mouth” or “mouth fungus”.
The critical fact to internalise: columnaris is bacterial, not fungal. Its cottony appearance fools the eye, but antifungal medications do nothing against it. Because some strains can kill a betta within 24–48 hours, treating it as a fungus and losing that first day is the most common fatal mistake owners make.
How Columnaris Progresses
| Stage | What you see | Urgency |
|---|---|---|
| Early | Pale or grey-white film on the mouth, lips, or a small “saddle” patch on the back; slightly frayed fin edges; subtle behaviour change | High — best treatment window |
| Established | Spreading cottony lesions, mouth erosion, ragged fins, clamped fins, reduced appetite, faster breathing | Critical |
| Advanced | Deep ulcers, severe mouth/jaw destruction, brown damaged gills, gasping, lethargy, refusing food | Often fatal — aggressive treatment, guarded prognosis |
Gill-form columnaris is especially dangerous because it can kill quickly with few external signs — a betta gasping at the surface with a faint film and no obvious body lesions may already have a serious gill infection.
What Causes Columnaris Outbreaks
- High temperature. Flavobacterium columnare multiplies far more aggressively in warm water, which is why cooling the tank (the opposite of ich/velvet) is part of treatment.
- Poor water quality. Ammonia, nitrite, and organic waste stress the fish and feed the bacteria.
- Physical injury and rough handling. Any break in the slime coat or skin is an entry point.
- Stress. Shipping, new tank mates, aggressive tank mates, and unstable conditions suppress immunity.
- Un-quarantined new fish or plants. The most common way a virulent strain enters a clean tank.
- Overcrowding. Increases stress and bacterial load simultaneously.
Setting Up the Columnaris Hospital Tank (Step-by-Step)
- Isolate immediately. Move the affected betta to a dedicated container the moment you suspect columnaris — it is highly contagious and fast.
- Match temperature first, then cool slowly. Transfer at the current temperature, then lower gradually (about 1°F per hour) toward 75–77°F to slow bacterial growth.
- Remove activated carbon. It will strip out the antibiotic.
- Add gentle aeration. Cooler water holds more oxygen, but if the gills are infected the fish still needs easy access to the surface and steady oxygenation.
- Add aquarium salt at about 1 teaspoon per gallon to reduce osmotic stress and support the slime coat.
- Begin the antibiotic without delay — every hour counts with this disease.
- Daily water changes with temperature- and salt-matched water to keep the bacterial load down.
Antibiotic Treatment in Detail
Columnaris requires a medication effective against gram-negative bacteria. A widely used and effective approach is a combination of kanamycin and a nitrofuran (furan-2) dosed together per their labels, which covers the bacterium from two angles. Some keepers successfully use doxycycline or minocycline-based products as alternatives. Whatever you choose:
- Dose for your true water volume, not the tank’s nominal size.
- Remove carbon and any chemical media first.
- Complete the entire course even if the fish looks better after a few days — stopping early invites a resistant rebound.
- Do not randomly stack extra antibiotics “to be safe”; follow a proven combination.
- Maintain the lowered temperature and salt throughout the course.
Recovery and Aftercare
If treatment is started early enough, improvement (lesions stop spreading, fish becomes more active and resumes eating) often begins within a few days. Once the full antibiotic course is complete and the fish is clearly stable:
- Slowly return the temperature to the normal 78–80°F range (about 1°F per hour).
- Clear residual salt and medication with a series of partial water changes over a week, then restore carbon and normal filtration.
- Feed small, high-quality meals to rebuild condition and support tissue repair, especially if the mouth was damaged.
- Disinfect any nets, siphons, and equipment that contacted the infected fish before reuse.
- Watch original tank mates closely for several days, since columnaris is highly contagious.
Severe mouth or gill damage may leave permanent scarring, but many bettas that survive columnaris recover well with attentive aftercare.
Why Columnaris Moves So Fast — and the Heat Mistake That Kills Bettas
What makes columnaris terrifying is its speed: a faint pale smudge at lights-out can be an open, spreading lesion by morning. Understanding why explains how to fight it. Flavobacterium columnare is a warm-water opportunist, and its virulence is directly tied to temperature. In the high-70s to low-80s°F — the exact range a betta needs — the bacterium reproduces aggressively and can go from a surface infection to a systemic, fatal one within 24 to 72 hours. This is the opposite of most fish diseases, where raising the temperature speeds the cure. With columnaris, heat fuels the pathogen.
This single fact is responsible for more dead bettas than the disease’s reputation alone. A keeper sees a fuzzy patch, reaches for the standard aquarium playbook — crank the heater to “boost the immune system” the way you would for ich — and unknowingly hands the bacterium ideal breeding conditions. Within a day the fish is far worse and the owner assumes the medication failed. In reality the treatment never had a chance, because the tank was a columnaris incubator. The correct move is the reverse: where the fish can tolerate it, gently lowering temperature toward the low-to-mid 70s°F slows the bacterium’s reproduction and buys the antibiotics time to work.
The second accelerant is organic load. Columnaris thrives in water heavy with dissolved organics — uneaten food, waste, a stale filter, an overstocked tank. A pristine tank does not stop an established infection, but a dirty one lets a minor wound infection explode. That is why the real treatment is always three things at once: an antibiotic effective against gram-negative bacteria (such as kanamycin or a combination targeting columnaris), aggressive water changes to strip out organics and bacterial load, and temperature management rather than a blind heat increase. Crucially, columnaris is highly contagious through the water column itself — not just direct contact — so the whole tank is infected the moment one fish shows symptoms, and treatment must be applied accordingly. Treat the water, control the heat, and act within hours, not days.
Sources & Further Reading
NippyFish — Cotton Wool Disease (Flexibacter columnaris); FishLab — Is It Fungus or Columnaris?; Betta Care Fish Guide — columnaris; Merck Veterinary Manual — Columnaris Disease (Flavobacterium).
Frequently Asked Questions
Is betta columnaris a fungus?
No. Despite looking cottony and being called “mouth fungus”, columnaris is a bacterial infection (Flavobacterium columnare) and needs antibiotics, not antifungal medication.
How fast does columnaris kill a betta?
Very fast — some fish die within 24–48 hours of symptoms appearing, which is why immediate antibiotic treatment is essential.
How do you treat columnaris in bettas?
Isolate the fish, slowly lower temperature to 75–77°F, use a gram-negative antibiotic (e.g., kanamycin + furan-2), add aquarium salt, and keep water pristine.
Should I raise or lower the temperature for columnaris?
Lower it. Columnaris thrives in warm water, so cooling to the low end of the betta range slows it — the opposite of ich and velvet.
How do I tell columnaris from fungus?
Columnaris is flat, filmy, grayish, and fast-moving (often on the mouth/saddle); true fungus is 3-D fluffy white tufts and slower. If unsure, treat for columnaris.
Is betta columnaris contagious to other fish?
Yes, highly contagious and fast-spreading. Isolate the affected betta immediately, watch all tank mates closely, and disinfect any shared nets, siphons, or equipment to stop it spreading through the tank.
Can a betta survive columnaris?
Yes, if it is correctly identified as bacterial and treated with a gram-negative antibiotic within hours of symptoms appearing. The prognosis drops sharply with delay or if it’s mistreated as a fungus, because virulent strains can kill in 24–48 hours.
What antibiotic treats columnaris in bettas?
A gram-negative antibiotic is required — a widely used effective combination is kanamycin together with a nitrofuran such as furan-2, with doxycycline or minocycline products as alternatives. Remove activated carbon first and complete the full course.
Why should I lower the temperature for columnaris but raise it for ich?
Columnaris bacteria multiply faster in warm water, so cooling to 75–77°F slows the infection. Ich and velvet are parasites whose life cycle you deliberately speed up with heat to reach their killable stage — opposite diseases, opposite temperature strategies.