Columnaris (Cotton Mouth): The Fast Killer Mistaken for Fungus
Columnaris kills faster than almost anything else in freshwater, and it is routinely treated as fungus — which does nothing, because it is bacterial. In warm water an untreated outbreak can go from first symptom to dead fish in 24 to 48 hours.
If you have a fish with a white or greyish patch and you are about to reach for an antifungal, read this first.
Columnaris vs. fungus
This distinction is the whole article.
- Texture. Columnaris looks flat, fuzzy-edged, and slightly grey or off-white — often described as mould-like film. True fungus is distinctly cottony with visible filaments standing off the body.
- Location. Columnaris favours the mouth (hence “cotton mouth”), the gills, the back just ahead of the dorsal fin (the classic “saddleback” lesion), and fin edges. Fungus usually starts at an existing wound.
- Colour. Columnaris patches often have a yellowish or brownish tinge at the margin. Fungus is white.
- Speed. Columnaris spreads visibly within hours. Fungus develops slowly.
- Body shape. A saddle-shaped lesion across the back is columnaris and nothing else.
When in doubt, treat for columnaris. Antibacterial treatment on a fungal infection is a delay; antifungal treatment on columnaris is a death sentence.
What it is
Flavobacterium columnare is present in most aquariums at low levels without causing problems. It becomes an outbreak when fish are stressed or injured and their protective slime coat is compromised.
Two factors drive severity dramatically:
- Temperature. The bacterium becomes far more virulent above roughly 80°F. This is why the standard “raise the temperature” advice for ich is actively dangerous here — it accelerates columnaris.
- Organic load. Dirty water, overcrowding, and high nitrate all favour it.
Treatment
Move fast.
1. Lower the temperature
Drop toward 75°F if your species tolerate it. This slows the bacterium significantly and is one of the few interventions that helps immediately. Lower gradually — no more than 2°F per hour.
2. Antibacterial medication
Columnaris needs a genuine antibacterial. Kanamycin and nitrofurazone are the commonly used options, and combining a gram-negative-effective antibiotic with furan-2 is a frequent approach for aggressive cases. Treat the whole tank — it spreads through the water.
Remove carbon filtration, which strips medication, and complete the full course.
3. Aquarium salt
Salt at around 1 tablespoon per 5 gallons supports the slime coat and adds mild antibacterial pressure. Reduce for scaleless species.
4. Water quality
Daily water changes to reduce bacterial load and organics. This meaningfully affects outcomes.
5. Increase aeration
Gill involvement is common and often what actually kills the fish.
Why fish die even after treatment
Columnaris frequently attacks the gills, and gill damage does not reverse quickly. A fish can clear the infection and still suffocate. This is why speed matters more here than with almost any other fish disease — a day of treating for fungus is often the difference.
Prevention
Keep temperature at the lower end of your species’ comfortable range in summer. Do not overstock. Keep nitrate down with regular water changes. Quarantine new fish, and handle fish gently — net damage to the slime coat is a classic entry point. Avoid moving fish between tanks unnecessarily.
Columnaris is opportunistic. It is already in your tank. What determines whether you ever see it is whether your fish are stressed enough to let it in.
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