Wintering
Fish Behavior in Cold Water
- At 10°C: Slowed metabolism, reduced activity
- At 8°C: Stop feeding them (they cannot digest food—it ferments in their intestines)
- At 4–5°C: Fish are nearly motionless, clustered in the deepest area (where the water is warmest)
- At 0°C: The surface water freezes, but the bottom, at 4°C, remains liquid
Requirements for Successful Wintering
- Minimum depth of 120 cm: The water at the bottom does not freeze
- Oxygenation Under the Ice: A small air pump or compressor to keep a hole in the ice open
- DO NOT chip away at the ice: The shock waves stun or kill the fish. Pour hot water on top to melt a hole.
- Remove leaves: It's dangerous to leave them there—they release gases as they decompose under the ice. Place a net over the pond.
- Filter: It can be turned off completely at temperatures below 5°C (bacteria go into hibernation). Restart it in the spring.
Goldfish vs. Koi in Winter
| Appearance | Goldfish | Koi |
|---|---|---|
| Lowest temperature | 0°C (under ice) | 2–4°C (preferably > 4°C) |
| Minimum depth | 80 cm | 120 cm |
| Food | Stop at 8°C | Stop at 10°C |
| General resistance | Major | Smaller (the fancy varieties don't overwinter well) |
Ponds in very cold regions (outside temperature < -10°C)
- "Pond heater" or "floating de-icer" — keeps a small area ice-free for gas exchange; it does not heat the entire pond
- Insulation cover over the pond
- For very small ponds: Move the koi to an indoor aquarium for the cold season