Diagnosing Fish Deaths in a 75-Gal Pond: Beyond Basic Tests

10 fish (koi/goldfish) in 75-gal pond, sluggish, bottom-sitting, deaths. Half water change, tests "fine" (pH, ammonia, nitrite, salt, chlorine). Dead bird found nearby. What’s next?

Test Kit Limitations

• Ammonia test strips/reagents expire in 6 months. False negatives common.
• **Test for Nitrate**—high levels (>80 ppm) cause chronic stress.
• Parasites (flukes, costia, trichodina) need microscope—scales raised 1–3 per parasite.
• Recent rain? Runoff brings fertilizer, pesticide, oil.

Stocking Density

10 fish in 75 gal = severe overcrowding. Even if "fine for 10 years," biofilter capacity maxed out. Waste accumulation eventually crashes system.

Action Plan

1. Re-test with **liquid reagent kits** (ammonia, nitrite, nitrate, pH, KH).
2. If any toxin >0: stop feeding, 30% daily water changes (dechlorinated) until zero.
3. If tests truly clean: **skin scrape/gill clip by vet or experienced keeper** for parasites.
4. **Reduce stock** to 3–4 fish max for this volume.

Bird carcass not a threat unless decomposing *in* water. Focus on water quality and parasites.