Guide
Columnaris: what it is and what to do
A fast-moving bacterial infection that looks like fungus - here's how to spot it, treat it, and prevent it.
- Not a fungus: Columnaris is caused by the bacterium Flavobacterium columnare - fungal treatments will do nothing.
- Symptoms: White or grey patches on skin, mouth, or fins; a saddle-shaped lesion behind the dorsal fin; rapid gill breathing; fraying fins.
- Speed matters: Virulent strains can kill fish within 24-72 hours of visible symptoms appearing.
- Treatment: Antibiotics effective against gram-negative bacteria - kanamycin, nitrofurazone, or doxycycline depending on what's available in your region.
- Root cause is almost always stress: Poor water quality, overcrowding, rough handling, or high temperatures weaken fish enough to let the bacteria take hold.
- Prevention: Quarantine all new fish, keep ammonia and nitrite at zero, and don't let temperatures climb above the comfort zone of your species.
What is columnaris?
Flavobacterium columnare is a gram-negative, filamentous bacterium that lives naturally in freshwater. It's present in most aquariums without causing any harm - the problem starts when fish are stressed or injured enough that their immune defenses can't keep it in check. Once it finds a foothold, it spreads fast.
The disease goes by several names: columnaris disease, cotton wool disease, saddleback disease, and mouth fungus. That last name is the one that causes the most confusion - the white, fluffy appearance around the mouth looks fungal, but the treatment approach is completely different. If you treat columnaris with an antifungal, you'll lose time you can't afford to waste.
In high-temperature tanks or with acute strains, fish can die within 24-72 hours of first visible symptoms - act the same day you notice something wrong.
Columnaris vs. true fungus: how to tell them apart
Both conditions produce white or greyish growths, but there are reliable visual differences once you know what to look for.
True fungus (Saprolegnia): Appears as a single, cotton-wool tuft - usually attached to a wound, dead tissue, or an unfertilised egg. The growth is fluffy and 3D, almost like a ball of fibres. It tends to be localised and doesn't spread across the body surface rapidly.
Columnaris: Tends to form a flatter, more diffuse patch - a band or saddle across the back, erosion around the lips, or a filmy grey coating on the fins. Under a magnifying glass, columnaris lesions often look more like a spreading discoloration than a raised tuft. The bacteria can also infect gills without producing any visible external patch at all, which is why gill columnaris gets missed until fish are already dying.
Columnaris and Saprolegnia can appear together on the same fish. The fungus often colonises tissue that columnaris has already damaged. Treat the bacterial infection first - if the white growth doesn't clear with antibacterials, a secondary antifungal course may be needed.
Recognising the symptoms
What you see depends on where the infection has established and how far it's progressed.
Skin and body lesions
Pale or white patches on the flanks, often with slightly raised or eroded edges. In the classic saddleback presentation, there's a grey or yellowish band across the back just behind the dorsal fin. This pattern is most common in livebearers like guppies and platys, and in catfish with softer skin.
Mouth lesions
White or grey erosion around the lips and mouth. The tissue looks like it's being eaten away. Fish may stop eating because it's painful. This is the "mouth fungus" presentation and it's one of the more common entry points for the infection in community tanks.
Fin involvement
Edges appear frayed, ragged, or receding - similar to fin rot. The difference is that columnaris lesions often show a whitish border at the active edge, and progression is faster. See the aquarium water parameters guide if you're trying to distinguish fin rot driven by poor water from a bacterial infection like this one.
Gill infection
This is the most dangerous presentation because there's little visible warning. Fish breathe rapidly, gasp at the surface, or hang near the filter outlet for oxygenated water. Gills may look pale, discoloured, or swollen. By the time you see these signs in multiple fish, you're already dealing with a serious outbreak.
Behaviour
Lethargy, loss of appetite, hiding, or sitting on the bottom. These are non-specific signs that accompany most serious infections, but combined with any of the above, they tell you to act immediately.
Most people wait a day or two to "see if it clears up." With a slow-strain columnaris infection that might work - but with the acute, high-temperature strains common in warm-water community tanks, that delay costs fish. If you see the saddle marking or mouth erosion, start treatment the same day.
What causes an outbreak?
The bacteria are already in your tank. What changes is your fish's ability to resist them.
Water quality
Elevated ammonia, nitrite, or nitrate stresses fish continuously, suppressing immune function and damaging gill and skin tissue - exactly the entry points F. columnare exploits. If you haven't tested recently, do it before anything else. Check your aquarium water parameters guide for the numbers you're aiming for. Ammonia and nitrite should be zero; nitrate below 20-40 ppm depending on species.
Temperature
This one catches people out. F. columnare becomes more virulent and replicates faster at higher temperatures. Tanks running above 24-26°C (75-79°F) see more severe outbreaks than cooler setups. This is why columnaris devastates guppy and molly tanks where people run the heat up. It's also why "treating" by raising temperature - advice that works for ich - is exactly the wrong move here.
Stocking and aggression
Overcrowded tanks, fish kept in the wrong social groups, or fish being bullied produce constant low-level stress. Even small nips and abrasions from fin-nipping tankmates create open entry points for the bacteria. Use a free stocking calculator to check whether your tank is genuinely understocked, or just looks it.
New arrivals and quarantine failures
Fish from a store already carrying the bacteria - showing no symptoms yet because they're under transport stress - get added to an established tank and trigger a full outbreak within a week. This is the single most preventable cause of columnaris.
Physical damage
Netting, transport, rough substrate, and sharp decorations all cause minor injuries that bacteria exploit. The slower you handle fish and the softer your net technique, the fewer entry points you create.
Most columnaris outbreaks happen in tanks that were already under water-quality or stocking pressure. Treating the infection and fixing nothing else means you'll be back here in a month. The disease is the symptom. The cause is almost always something upstream in your tank management.
Treatment
Columnaris is a bacterial infection, so you need an antibacterial that works against gram-negative bacteria. The right choice depends on what's available where you live.
Antibiotic options
Kanamycin sulfate - effective against F. columnare and available in the US as a standalone medication. Often the first recommendation for serious infections.
Nitrofurazone - broad-spectrum antibacterial, available in combination products. Works well for external lesions, less effective for gill infections.
Doxycycline - an option where other antibiotics aren't available over the counter; check dosing guidance specific to your region.
Furan-2 / Furazolidone combinations - available in some markets, useful for mild to moderate cases.
In the UK and EU, most dedicated fish antibiotics require a vet prescription. In practice, hobbyists there often use antibacterial medications based on acriflavine or similar compounds for milder cases, but severe outbreaks may genuinely need a vet consult and a prescription.
How to treat
Isolate affected fish in a hospital tank if possible. This lets you dose accurately, avoid disrupting the biological filter in your main tank, and monitor the sick fish closely. If the entire tank is affected, treat in-place - but be aware that standard antibiotic doses will damage your beneficial bacteria. Run extra aeration, test daily, and do small water changes to manage any ammonia spike.
Follow the full treatment course. Stopping early because the fish looks better is how you breed antibiotic-resistant bacteria and set yourself up for a relapse.
Do a 25-30% water change before starting treatment to reduce organic load. Remove activated carbon from your filter - it adsorbs medication and makes the dose ineffective.
Salt as a supportive measure
Adding aquarium salt (sodium chloride) at 1-3 g per litre can help reduce osmotic stress and slow bacterial spread. It's not a cure on its own, but it's a reasonable supportive addition for fish like livebearers that tolerate it well. Don't use it with scaleless fish or sensitive species like cory catfish.
Temperature during treatment
Lower, don't raise. If your tank is running above 24°C (75°F) and your species can tolerate it, dropping to 21-22°C (70-72°F) slows bacterial replication and buys time. The bacteria are most virulent between 25-35°C (77-95°F).
If losses keep happening after treatment, check whether your nitrogen cycle is actually established. An uncycled or crashed cycle produces the ongoing water quality stress that keeps columnaris coming back. Read the nitrogen cycle explained if you're not sure where yours stands.
Prevention
Quarantine - always
Four weeks minimum for all new fish before they go into your main tank. Most columnaris introductions are preventable with this step alone. A 10-15 litre bare-bottom tank with a sponge filter is all you need.
Water quality
Zero ammonia, zero nitrite, nitrate under control. Test weekly, not monthly. If you're doing water changes reactively rather than on a schedule, read how often to do water changes to build a routine that keeps parameters stable before problems start.
Avoid temperature stress
Keep your tank within the natural range for your species and don't let it fluctuate. Sudden temperature swings - from a heater malfunction, a cold water change, or room temperature changes in winter - stress fish immediately and create vulnerability windows.
Stock sensibly
Overcrowding is a slow-burning stressor. So is keeping fish with incompatible aggression levels. If you're building a new community tank, check the best fish for beginners guide for combinations that actually work together peacefully.
Handle fish gently
Wet your hands before touching fish. Use a soft, fine-mesh net. Move fish in water where possible rather than lifting them through air. Every abrasion is a potential entry point.
App-aquatic lets you log water test results over time so you can spot gradual parameter drift before it causes a disease outbreak - not after. Consistent records are the easiest early-warning system you have.
If fish keep dying
If you've treated for columnaris and fish are still dying, or if losses continue after the outbreak appears to be over, there may be a concurrent infection (columnaris and ich together is not unusual), a water quality problem that treatment didn't address, or a misdiagnosis. Review your water test history, check for other symptoms, and consider whether the original diagnosis was correct.
Track your parameters and get care reminders with App-aquatic.
Get the free appCan columnaris heal on its own without treatment?
Occasionally a very mild, chronic-strain infection will stabilise if water quality is improved significantly and stress is removed. But you're gambling on strain virulence you can't know in advance. With the acute strains common in warm tanks, waiting without treatment usually means dead fish within days. Treat and fix the water - don't wait to see what happens.
Is columnaris contagious to other fish in the tank?
Yes. F. columnare spreads through the water column and via direct contact. Once one fish is showing symptoms, assume the rest have been exposed. Isolate visibly sick fish to a hospital tank if you can, but treat the main tank too if multiple fish are affected.
Why is my fish getting columnaris repeatedly?
Recurring columnaris almost always points to a persistent stressor - usually water quality that never fully recovers between outbreaks, or chronic overcrowding. The bacteria live in the tank permanently; your fish's immune function is what changes. Fix the underlying condition rather than cycling through treatments.
Can I use aquarium salt instead of antibiotics?
Salt alone won't clear an established columnaris infection. It reduces osmotic stress and may slow mild surface lesions, but it doesn't kill the bacteria at concentrations safe for freshwater fish. Use it as a supportive measure alongside proper antibacterials, not as a replacement.
Will raising the temperature help treat columnaris?
No - this is the opposite of what you want. F. columnare is most virulent and replicates fastest between 25-35°C (77-95°F). If anything, drop the temperature slightly during treatment if your fish can tolerate it. Never raise heat to fight columnaris the way you might for ich.
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