Columnaris
Symptoms
White or grayish-yellow patches around the mouth, gills, or back — often resembling a fuzzy 'saddle' across the dorsal surface (giving the disease one of its names). Frayed fins, lethargy, gulping at the surface. Gill infection is the fast killer: fish can die within 24 hours with no visible body lesions. Often misdiagnosed as fungus because of the cottony appearance, but Columnaris is bacterial.
Causes
Flavobacterium columnare, a gram-negative rod that thrives in warm water (above 80°F) with elevated ammonia and organic load. Triggered by overcrowding, poor water quality, and stress. Highly contagious between tanks via nets, water transfer, and asymptomatic carriers.
Treatment
Lower temperature to 75°F if the species tolerates it (slows the bacteria, the opposite of ich treatment). Heavy water change. Treat the whole tank with kanamycin OR a sulfa-based antibiotic. Adding salt at 1 tsp/5gal helps inhibit the bacteria in freshwater. Acute outbreaks require fast action — 48 hours from first symptoms can be fatal. Remove activated carbon while medicating.
Prevention
Avoid heat-stressing tropical fish above their normal range. Keep nitrate low. Quarantine new arrivals. Don't share nets between tanks without disinfecting.
Notes
Easy to confuse with true fungus (Saprolegnia). Fungus is whiter and cottonier; Columnaris is yellow-gray and flatter. When in doubt, treat as Columnaris — fungus rarely kills as fast and bacterial treatments don't make it worse.