Moving Indoor Fish to Outdoor Pond in Winter: Certain Death

Temperature Shock Kills Tropical-Acclimated Fish

Small koi and goldfish from an indoor tank (70–75°F) cannot be placed directly into an outdoor pond in winter (35–45°F). The 30°F+ difference causes fatal cold shock.

Correct Transition Protocol

1. Keep indoors in a heated aquarium until pond temperature reaches 60°F+ (late spring).
2. Gradual acclimation: Float bag 30 min, then add pond water to bag over 1 hour.
3. Match temperature within 2°F before release.
4. Quarantine 2–4 weeks to prevent disease introduction.

If You Must Overwinter Indoors

• Tank: 100+ gallons for koi, 30+ for goldfish.
• Filtration: Canister or sump rated for 3x volume.
• Temperature: 65–70°F stable.
• Light cycle: 10–12 hours.

Do not risk it. Wait for spring.