Winter Care Guide for Medaka | Outdoor Cultivation Tips

Medaka are hardy fish well-adapted to Japan's four seasons. They can survive summer heat and winter cold—this image itself is true, but many people make the mistake of assuming they can "survive winter without doing anything." It's not uncommon to find the entire population gone when spring arrives.

For medaka to successfully survive the winter, it's important to prepare their environment before water temperatures drop. This guide breaks down overwintering preparation into easy-to-follow steps that even first-time medaka keepers can implement.

Understanding How Medaka Overwinter

Medaka are cold-blooded animals, so as water temperature drops, their body temperature falls and their activity level decreases dramatically. When water temperature falls below 10°C, their movements slow noticeably, and below 5°C they enter a "dormant state," remaining motionless at the bottom of the container.

This state is not "death-like" in the true sense, but rather an energy-saving mode that minimizes energy consumption. With their metabolism slowed, they no longer need to eat, and digestive activity nearly stops.

Even if water surface freezes thinly, the bottom water is unlikely to fall below 0°C, allowing medaka to survive. However, they cannot survive if the entire container freezes solid. The key to overwintering care boils down to two points: "prevent the bottom from freezing" and "avoid sudden temperature changes."

There are slight differences in cold tolerance between medaka varieties, but the basic concepts of overwintering care apply to all types. Our medaka selection features many individuals from domestic breeders with proven overwintering track records, so check it out when choosing your variety.

Timing and Method for Stopping Feeding

The first step in overwintering preparation is managing food.

When water temperature drops below 15°C, reduce the number of daily feedings and feed smaller amounts. When water temperature falls to 10°C or below, stop feeding completely.

Feeding cessation is necessary because medaka's digestive organs also slow down in low temperatures. If uneaten food remains in the intestines, it can cause intestinal blockage. Additionally, leftover food decays in the water, causing rapid water quality deterioration.

• 15–12°C: Feed 2–3 times per week, in small amounts
• 12–10°C: Feed once per week, minimal amounts
• Below 10°C: Stop feeding entirely

When spring arrives and water temperature rises above 10°C again, gradually resume feeding in small amounts. The digestive system after overwintering isn't fully functional yet, so avoid giving large amounts suddenly.

Container Selection and Water Volume

Choosing the right overwintering container directly affects medaka survival rates.

Types of Containers

Styrofoam containers are ideal for overwintering. They have excellent insulation, keeping water temperature changes gradual even as external temperatures drop, and freeze-resistant. Many supermarkets and fish vendors give them away free, making them economical too.

If using plastic containers, place them inside a styrofoam box or wrap the outside with bubble wrap—either method provides good insulation.

Water Volume Guidelines

The more water you have, the more gradually temperature changes occur. With very little water, nighttime cooling can cause dramatic temperature drops, so adjust water level higher before winter arrives.

• Ensure a minimum of at least 10L of water
• For every 10 medaka, aim for about 20L
• If water level drops from evaporation, add dechlorinated water gradually in small amounts

Placement Location and Freeze Prevention

Move to a location protected from wind

Cold winter winds cause sharp drops in water temperature. If keeping containers outdoors, choose a location with wind protection.

• Under eaves: Protected from direct rain and snow, with wind shelter
• Near a garage entrance: Good balance of sunlight and wind protection
• South side of a fence or wall: Sheltered from north wind, receives daytime sunlight

If north winds hit directly or temperatures drop extremely in cold regions, consider moving containers indoors to an entrance or similar location.

About surface ice

If water surface freezes thinly but the bottom remains unfrozen, your medaka are alive. Don't force the ice open. The impact of breaking it and sudden temperature changes can stress the fish more.

However, if there's risk of complete container freezing, place a styrofoam lid on top or move the container indoors.

Water Changes and Water Quality Management

Winter water changes are basically unnecessary

During overwintering, medaka's metabolism is so low that they produce little waste and water becomes dirty slowly. Unnecessary water changes only stress the fish and can cause temperature shock.

Avoid water changes in winter as a rule—only add water to replace evaporation. Use only dechlorinated water for top-ups, and add it gradually if temperature differs significantly to minimize thermal shock.

Prevent fallen leaves and debris

Autumn through winter is leaf-fall season. When leaves enter the container, their decomposition releases ammonia and water quality deteriorates.

• Cover the container with bird netting or non-woven fabric
• If leaves get in, remove them gently without startling the fish
• Algae and moss help water filtration, so you needn't remove them forcefully

Summary: Prepare for Reunion in Spring

Medaka overwintering requires no special equipment or complex techniques. When you focus on the key points, it's straightforward:

1. Stop feeding when water temperature reaches 10°C or below
2. Ensure sufficient water volume and use styrofoam containers
3. Move to a windless location and prevent freezing
4. Minimize winter water changes—only top up with water
5. Protect with netting to prevent fallen leaves and debris

If you prepare these in autumn, your medaka will make it through to spring on their own. During winter, observe quietly and remember that avoiding unnecessary stress is also important care.

When spring arrives and water warms, and your medaka begin swimming actively throughout the container—the joy of that morning awaits. To properly prepare for spring 2026 or to welcome new medaka, explore the medaka selection and find healthy, breeder-direct individuals.