Lethargic Koi in 1,500-gal Pond: Water Quality and Parasite Check

1,500-gal pond, 10 koi. One orange metallic koi (20 cm) stunted vs. sibling (35 cm). Now lethargic, off food, rests on bottom/surface, allows touch. Asks advice.

Systematic Diagnosis

1. Water test: pH, hardness, alkalinity, salinity, O₂, ammonia, nitrite, nitrate, heavy metals.
2. 10% water change + dechlorinator + 0.05% salt (0.1% if no plants).
3. Clean filters, add fresh carbon.
4. Skin scrape for parasites (koi club does free).

Further Steps

If not water/parasites: bacterial/fungal/viral. Try MelaFix in pond or antibiotics in quarantine. Injected antibiotics need vet. Lab diagnostics available (US). See Koivet.com.