Columnaris in aquarium fish — telling it from fungus and treating it
A bacterial disease often mistaken for fungus. Greyish-white cotton-like patches on head and fins; the aggressive strain can kill in 24 hours. Treatment — antibiotics, lower temperature, salt.
Columnaris is caused by the Gram-negative bacterium Flavobacterium columnare. The disease is often called 'false fungus' — it looks like saprolegnia but is entirely different in nature and needs completely different drugs. Misdiagnosis is the leading reason fish die from columnaris.
Symptoms: grey-white 'cotton' patches on the head, back, fin bases; ulcers and erosion; soft frayed fins; a characteristic 'saddleback' necrosis across the back; rapid breathing if gills are involved. The aggressive strain above 28 °C is particularly dangerous — death in 24 hours.
Treatment and how to tell from fungus
Key difference: fungus is fluffy, long fibres that come off easily with tweezers. Columnaris is a dense coating fused into the skin and won't peel off. Treatment: kanamycin 50 mg/L or erythromycin/oxytetracycline in a hospital tank, drop temperature to 24–26 °C (the bacterium multiplies slower), salt 1 g/L, full water change after 24 hours.
Prognosis and prevention: with early treatment and lower temperature, survival is 60–80%. Main triggers — high temperature, poor water, injuries from aggressive tankmates. Quarantine new fish without exception — columnaris often arrives from overcrowded shop tanks.
FAQ
- Can columnaris be cured without antibiotics?
- Mild cases — salt 1 g/L + dropping temperature to 24 °C sometimes stops progression. But with typical 'cotton' patches an antibiotic is essential — without it the fish dies.
- Is columnaris contagious to tankmates?
- Yes, especially at high temperature and under stress. Isolate the sick fish and preventively drop the main tank to 26 °C for a week.
- Why do bettas and labyrinth fish so often get columnaris?
- They're prone to fin damage (aggression, wrong tankmates), and Flavobacterium colonises damaged tissue first. Give them calm tanks and clean water.
Scientific board — ichthyologists and veterinarians
Ichthyologists and veterinarians with university degrees · Reference FishBase, Seriously Fish and peer-reviewed literature · Sign every reviewed article with their credentials shown
PhD in ichthyology, researcher of African Great Lakes cichlids
PhD in ichthyology, University of Edinburgh · Field research in Malawi, Tanganyika and Victoria (2013–2018) · 12+ peer-reviewed publications on cichlid behaviour
Sources
- Seriously Fish — Columnaris · Seriously Fish · 2026-05-31
- FAO — Fish disease guide · FAO · 2026-05-31