Koi and other hardy pond fish can remain outdoors during winter when the pond provides suitable overwintering conditions. The most important requirements are maintaining oxygen levels, allowing trapped gases to escape, and avoiding disturbance of the warmer water near the bottom.
Maintain an opening in the ice
If your pond freezes over, maintain an opening in the ice to allow proper gas exchange.
- Use a pond aerator or small recirculating pump to create gentle movement near the water's surface.
- Position the aerator or pump on an upper pond shelf, rather than at the bottom, to avoid disturbing deeper water where fish overwinter.
- In areas experiencing prolonged freezing, use an Aquascape Pond De-Icer to help maintain an opening in the ice.
- Keep the aerator and floating de-icer apart. Water movement from the aerator can carry heated water away from the de-icer, causing it to operate more frequently.
Reduce stress on overwintering fish
- Remove excess leaves and organic debris before the pond freezes.
- Allow fish to remain undisturbed in deeper pond areas.
- Stop feeding koi and most other common pond fish when water temperatures remain below 50°F (10°C).
- Never strike or break pond ice with force, as underwater vibrations can harm fish.
Important: An aerator may not maintain an opening during extreme freezing conditions. Check the pond regularly and use a pond de-icer when additional protection is needed.