New Koi Dying in Established Pond: Quarantine and Water Testing

Why New Fish Die While Old Ones Survive

Established fish are adapted to your pond's specific chemistry (pH, hardness, microbial flora). New koi—especially large ones—suffer osmotic shock and immune suppression when moved into different water, making them susceptible to latent infections.

Essential Protocol for New Additions

1. Quarantine 3–4 weeks in a separate tank with cycled filter, matched temperature, and salt (0.1%).
2. Test source water and pond water for pH, KH, GH, ammonia, nitrite, nitrate, chlorine.
3. Acclimate slowly: 1–2 hours of drip acclimation.
4. Observe for symptoms (flashing, clamped fins, lesions) before release.

If Fish Are Already Dying

• Test immediately for ammonia/nitrite spike from the new bioload.
• Do a 30% water change with dechlorinator.
• Add pond salt (0.05% with plants).
• Increase aeration.