(This is based on my own experience. There are many valid approaches, so please read this as reference only.)
Before winter really starts (around mid-November in North Carolina)
- I don’t wait until it’s freezing. Once fall settles in, I do a full reset.
- I clean the container and replace the water.
- This helps remove predator larvae and starts winter with clean, stable water.
- I always do this before it gets cold enough that I don’t want my hands in the tub.
Feeding going into winter
- As water temperatures drop below about 60°F, I start tapering food.
- When temps approach 50°F, feeding slows dramatically.
- I only feed during the warmest part of the day, never in the morning or late evening.
- I transition away from heavy, high-protein foods.
- Once true winter arrives, I mostly stop feeding.
- If there’s an unusually warm day and the fish are clearly active, I may offer a very small amount, but I never let food sink and sit.
What I avoid doing in winter
- I don’t break surface ice.
- I don’t try to wake the fish up or force activity.
- Medaka sitting still near the bottom in cold water is normal behavior.
Aeration and equipment
- Once feeding drops, I turn off aeration.
- Cold water holds more oxygen, and current only makes fish burn energy.
- Less movement helps them conserve strength through winter.
- During winter, I keep equipment very minimal, sometimes none at all.
Container size and depth
- I’ve learned that water volume and depth matter more than almost anything else.
- I aim for containers around 27 gallons or more, with real depth.
- Larger volumes change temperature more slowly and are much less likely to freeze solid.
- Thin surface ice is fine. A container freezing solid is not.
If I can’t use large containers
- I use a foam (styrofoam) box for insulation.
- Bigger water always beats heavier insulation with less water.
Extra insulation tricks I use
- I float a piece of styrofoam on the water surface to reduce heat loss and make ice less likely to form.
- I sometimes place a corrugated clear panel over the top to block wind, rain, or snow while still letting light through.
- Styrofoam and clear panels don’t affect water quality, so they’re safe to use.
Stocking density in winter
- I stock much lighter than in summer.
- A rough guideline I follow is about 1 fish per gallon or less, so in a 27-gallon container, I usually keep it under 27 fish.
- In winter, one loss can quickly affect the whole container if density is too high.
Water choice
- I prefer green water for winter.
- It’s stable, gentle on fish, and tends to hold better through the season.
- If the container already has green water, I keep it.
- If not, I prepare it ahead of time and add it during the reset.
Placement of containers
- Full-day direct sun can cause large day-night temperature swings, especially in smaller volumes.
- Too much shade can cause green water to collapse.
- I usually aim for partial sun, adjusting with shade cloth if needed.
- Every yard is different, so placement takes some trial and error.
Water changes during winter
- Once winter starts, I don’t do regular water changes.
- I mostly just top off evaporated water.
- In early winter, if I’m still feeding a little, I may do very small changes.
- Otherwise, I leave things alone as much as possible.
Plants and leaves
- Before winter, I remove most existing plants, including water lettuce and other floating or rooted plants.
- Cold weather slows plant growth, and dying plants can rot and foul the water.
- The one exception for me is persimmon leaves.
- I add a few rinsed persimmon leaves to the container.
- They’re commonly believed to have mild antibacterial properties and hold up well through winter.
- The leaves also give the fish a calm place to rest underneath.
- I don’t rely on them as a treatment, just gentle, natural support.
Small fish and fry
- For outdoor wintering, I feel safest with medaka around 0.8 inches (2 cm) or larger.
- I’ve overwintered smaller fish, but success drops quickly.
- If they’re below that size, I usually bring them indoors.
Indoor wintering
- Indoors is easier because water won’t freeze.
- Even indoors, if water stays below 60°F, feeding slows.
- I feed lightly and watch leftovers closely.
- Overfeeding causes more problems than underfeeding.
Daruma types
- I overwinter daruma types outdoors as well.
- I haven’t found them dramatically weaker, but I monitor them closely.
- If I see buoyancy or flipping issues, I isolate immediately.
- One struggling fish can quickly impact water quality for the entire container.
Final thoughts
- Winter success isn’t about doing more. It’s about preparing well, then leaving them alone.
- There are many ways to overwinter medaka successfully.
- What works for me may not be ideal for everyone.
- Always adjust based on your climate, container size, and experience.
