Spring Koi Deaths: Diagnosing Recurring Losses

A 13,000-gallon pond loses most koi each spring for four years. Fish show internal bleeding—reddening under skin, blood-filled blisters. Setup includes mechanical filter (12,000 L/hr), aeration, 1,000-gal plant filter, 6-ft depth. Owner asks about salt dosing (0.05% solution) for 13,000 gallons.

Why Spring Is Critical

Post-winter, koi are immunocompromised. Stress opens the door to bacteria, parasites, and fungal infections. The described symptoms (hemorrhaging, blisters) suggest severe bacterial septicemia or parasitic damage, but microscopic examination is essential.

Diagnostic Checklist

1. Full water panel: pH, GH, KH, ammonia, nitrite, nitrate.
2. Determine if losses affect one or multiple fish.
3. Document visual signs: lesions, flashing, clamped fins, isolation.
4. Skin scrapes & gill biopsies under microscope for parasites.

Salt Use: Proceed With Caution

Therapeutic salt levels range 0.1–0.3%. For 13,000 gal (≈49,210 L):

• 0.1% = ~49 kg salt
• 0.3% = ~148 kg salt

However, salt can further stress weakened fish. Test water first. Consult a koi veterinarian or experienced dealer for targeted treatment. Blind dosing risks killing more fish.

Priority: Get a live, affected fish to a professional for scraping and diagnosis before any treatment.