White Fuzzy Spot on Koi: Fungus, Salt Dosage, and Treatment Options

Diagnosis: Likely Fungal Infection (Saprolegnia)

A fuzzy white patch on the head often starts at a minor injury (scrape, parasite attachment). At 45°F, koi immune systems are slow, allowing fungus to establish.

Salt: You're Using 10x the Recommended Dose

Therapeutic salt for ponds is **0.1–0.3%** (1–3 lbs per 100 gallons). **2% is extremely high**—near seawater salinity. It stresses freshwater fish and may damage kidneys. Reduce immediately via water changes to 0.3% max.

Treatment Plan

1. Lower salt to 0.3% with partial water changes.
2. Add PimaFix (API) – natural antifungal, safe for plants at label dose. Aerate well.
3. If no improvement in 5 days: Consider a formalin/malachite green treatment (e.g., ProForm-C) in a quarantine tank. Do not use in main pond with plants.
4. Improve nutrition: Soak pellets in garlic extract (immune booster) once water warms above 50°F.

Quarantine Is Ideal

If you can net the koi (use a sock net, not a stiff net), treat in a heated hospital tank (65–70°F) where medications work faster and fungus grows slower. A 4-ft deep pond makes netting hard—consider a seine net or drain partially.

Fungus is treatable if caught early. Fix the salt level first.