Goldfish Dying Slowly: Mystery in 4,000-L Pond

4,000-L pond, biofilter, goldfish + 3 koi. Goldfish dying 1–2 at a time over 2–3 weeks. Water changes, plant removal, bacterial/fungal treatments, algae treatment done. pH 7.5, salts OK. Water clear. Autopsy shows no visual symptoms. Fish "sad" day before death.

Systematic Approach Needed

Chronic low-level mortality with no gross signs suggests:

• Sublethal ammonia/nitrite (test at different times of day)
• Gill parasites (Costia, Trichodina—require scrapes)
• Viral (KHV, SVC—PCR test needed)
• Chronic bacterial septicemia (internal, no external signs)
• Heavy metals (copper, zinc from plumbing)

Immediate Steps

1. Stop all chemicals (algaecides stress fish).
2. Test ammonia/nitrite twice daily for 3 days.
3. Get gill/skin scrapes on a live, "sad" fish.
4. If parasites found: Treat specifically.
5. If water clean & no parasites: Consider viral testing or heavy metal panel.

Microscopy is the only way to stop trial-and-error. Find a koi vet or experienced hobbyist with a scope.