Treating White Patches and Lethargy in a Koi: Fungus, Columnaris, or Injury?

Differential Diagnosis

• Color change: Koi naturally change pattern, but sudden white patches + lethargy suggest pathology.
• Ich (white spot disease): Shows as many tiny salt-grain dots, not patches.
• Fungus (Saprolegnia): Cottony white tufts, often on injuries.
• Columnaris (Flavobacterium): White/gray patches, saddleback lesions, frayed fins; highly contagious.
• Injury: Spawning scrapes, predator strikes, or rubbing on objects can become infected.

Recommended Treatment

1. Quarantine: Move the koi to a separate tank to reduce medication cost and stress on other fish.
2. Salt: 1 tablespoon per 3 gallons (0.3% salinity) – osmoregulatory aid, antiseptic.
3. MelaFix: Natural antibacterial (tea tree oil) – follow label.
4. Antibiotic: Erythromycin (e.g., Maracyn I) for gram-positive bacteria like Columnaris. Use in quarantine only.

See fish treatment guide for details. Many koi recover from severe infections with prompt care.