Indoor Tank Winter
For indoor tanks, winter changes little. Keep temperature stable at 18–22°C with a heater if room temp drops below 15°C. Many indoor goldfish never experience real winter and live their whole lives at 20°C.
Some keepers deliberately drop indoor temp to 12–15°C in winter to mimic seasons — this triggers natural breeding behavior in spring.
Outdoor Pond Winter
In freezing climates (where the pond surface ices over), goldfish enter semi-dormancy. Their metabolism drops dramatically. They stop eating around 10°C and cluster near the bottom.
Pond depth must be enough that the bottom remains above freezing — typically 60+ cm (2 ft) for mild winters, 90+ cm (3 ft) for harsh winters.
Managing Ice Cover
Total ice cover is dangerous because gas exchange stops. Toxic gases build up under the ice. Always keep at least one ice-free hole.
Use a pond de-icer (floating heater that keeps a small area open), an air pump with stone, or a small surface bubbler. Never break ice with a hammer — the shock can kill fish.
Winter Feeding Schedule
Stop feeding when water drops below 10°C. Goldfish digestion stops at this temperature, and undigested food rots inside them. Fish lose weight slightly but live off body reserves.
In late autumn, switch to wheat-germ-based winter pellets that digest at lower temperatures. Resume normal feeding gradually in spring as temperatures rise above 10°C.
Spring Wake-Up
Watch for signs of stress: white film, fungal patches, lethargy. Spring is when bacterial and fungal infections strike weakened fish. Test water immediately when fish become active.
Resume filter operation if it was reduced for winter. Begin water changes once weekly.