Koi Dying Weekly: Fungus as Secondary Infection

Situation

1000-gal established pond, koi dying weekly. Fungus seen. Treated with 5-day antifungal. Fish not eating well. One fish infecting others?

Key Insight

Fungus (Saprolegnia) is almost always secondary to a primary stressor: parasites (flukes, ich, costia), bacterial infection, poor water quality, or injury.

Diagnostic Steps

1. Microscope skin/gill scrapes by experienced keeper/vet → identify parasites.
2. Test water: ammonia, nitrite, nitrate, pH, KH, DO.
3. Observe behavior: flashing, clamped fins, piping, isolation.

Treatment Protocol

1. Treat primary cause (e.g., praziquantel for flukes, formalin/malachite green for ich/costia).
2. Antifungal (e.g., methylene blue, potassium permanganate) only after primary addressed.
3. Improve water quality: water changes, reduce feeding, add salt (0.3%) if no plants.

Do Not

Repeat antifungal alone—it fails because primary issue persists.