Medaka Outdoor Winter Guide — Water Changes, Feeding & Freeze Prevention

Medaka are native Japanese fish that can overwinter outdoors, but captive conditions differ from the wild. Proper preparation and management are essential for safe outdoor overwintering.

Winter Medaka Behavior

Medaka are ectothermic. Below 15°C, appetite decreases. Below 10°C, they become still near the bottom. Below 5°C, they enter near-hibernation with minimal movement. This is normal overwintering behavior, not illness.

Preparation (October-November)

• Container check: Ensure minimum 10 liters of water. Small containers have volatile temperatures and freeze more easily
• Maximize water level: More water means slower temperature changes and reduced freezing risk
• Clean the bottom: Remove accumulated debris before winter. Do a 1/3 to 1/2 water change, avoiding full replacement
• Move weak fish indoors: Thin, unhealthy, or undersized fry (under 1cm) should be kept indoors with a heater

Winter Management (December-February)

• Feeding: Stop feeding when water drops below 10°C. Undigested food rots inside the fish. Feed tiny amounts only on warm days (15°C+)
• Water changes: Generally avoid winter water changes. Temperature shock from fresh water is a bigger risk. Top off with temperature-matched water as needed
• Covers: Use netting or screens to block falling leaves and reduce wind cooling

Freeze Protection

Medaka survive surface freezing as long as the bottom doesn't freeze.

• Polystyrene insulation: Wrap containers with foam boards. Polystyrene containers already have good insulation
• Lids: Foam board or corrugated plastic lids block cold air. Leave gaps for oxygen