Columnaris (Flavobacterium columnare) is one of the most aggressive freshwater fish diseases - the disease can kill multiple fish within 48 hours of visible symptoms. Treatment protocol: Move infected fish to a hospital tank with mature filtration. Dose Kanaplex per manufacturer instructions for 7-10 days. Daily 25% water changes with parameter-matched water + dechlorinator. Re-dose Kanaplex after each water change. Alternative for milder cases: Furan-2 5-7 day course. Prevention: Avoid stress (overcrowding, sudden parameter changes, aggressive tankmates). Quarantine new fish 14-21 days. Visual signs: White or grey saddle-shaped patches on the back, mouth fungus appearance, frayed fins. Often confused with fungus but spreads faster. See the full columnaris guide for symptoms checklist + alternative treatments.
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