Columnaris is caused by Flavobacterium columnare and primarily affects skin and gills. Lesions can look white, grey, yellow-brown or cottony, so the disease is often confused with fungus. Mouth erosion, saddle-like skin lesions, fin damage and rapid breathing can occur. Because severe cases can progress fast and treatment depends on whether disease is external or systemic, prompt water-quality correction and fish-veterinary or diagnostic advice are preferable to guessing with random medications.
The word 'cottony' makes many fishkeepers think fungus, but columnaris is bacterial. This distinction matters because the wrong treatment can waste valuable time in a disease that may advance quickly.
Common signs
- Pale, grey or yellowish patches on skin or around the mouth.
- Slimy or cotton-like surface material over damaged tissue.
- Fin erosion, ulcers or necrotic skin.
- Rapid breathing or gill damage.
- Fast spread through stressed groups of fish.
Why diagnosis can be difficult
Columnaris lesions can resemble Saprolegnia fungus or trauma. Merck notes that a presumptive diagnosis may be supported by wet-mount examination, while culture can confirm the organism.
Visual appearance alone is therefore not always enough to choose medication confidently.
Environmental factors matter
High organic load, crowding, injury and stressful handling can increase risk. Warm conditions can favour disease expression in many susceptible warmwater species.
Correct ammonia, nitrite, oxygenation and sanitation immediately while you seek a diagnosis.
Treatment should match the disease stage
External disease and systemic disease are not managed identically. Veterinary references describe different options depending on severity and species, so a one-size-fits-all bottle recommendation is inappropriate.
Do not combine treatments unless compatibility is known.
Prevention
- Quarantine new fish.
- Reduce organic waste and avoid overstocking.
- Minimise skin damage during transport and netting.
- Do not share wet equipment between tanks without disinfection.
Frequently asked questions
Is columnaris a fungus?
No. It is a bacterial disease caused by Flavobacterium columnare.
Can columnaris look like cotton wool?
Yes. Surface exudate can look cottony, which is one reason it is confused with fungal disease.
Is columnaris always slow?
No. Some outbreaks can progress rapidly, especially when fish are stressed or gills are involved.
Sources and further reading
This guide separates general aquarium arithmetic from fish-welfare decisions. For species-specific limits, always check the needs of the fish you intend to keep.