Overwintering young fish: will your medaka reach 1.5 cm?

Every autumn the same question comes back: do you let late fry overwinter, or do you bring them inside? The days get shorter, the water cools down and the last spawns of the season are often still small. The 1.5 cm rule gives you a yardstick so that you do not have to make that choice on gut feeling.

Below you can read why the size of a young medaka (Oryzias latipes) says so much about its chances, how to measure it without stress, and what to do if it stays too small for the cold.

Why the size of juveniles matters so much

At first glance a medaka of 1 cm and one of 2 cm look hardly different. In stored energy, the difference is a multiple: the volume of a fish grows with the cube of its length, so a fish that is twice as long carries about eight times as much reserve.

Surface area relative to volume plays a role too. A small body loses relatively more heat, and therefore more energy per hour. What a large fish has left over is gone in a small one.

When the water drops below 10 °C, the metabolism of medaka nearly stops. They eat little to nothing and live off what was built up in the preceding weeks. What hasn't grown in September won't grow any more in December.

Juveniles from a second or third spawn never had those growing months. Where a May fish has been foraging since June, a fish from August only has September and part of October to fill up.

The 1.5 cm rule: how to measure juveniles in October

The rule circulating here is short: if your juveniles are shorter than 1.5 cm on 1 October, bring them indoors. If they are longer, they can in principle stay outside. This date was not chosen by chance. Around early October the water in a tub or shallow pond structurally drops below 15 °C, and from that point growth slows so much that a deficit can no longer be made up. That's how you decide whether to dare to overwinter young fish or not.

Don't measure with a tape measure along the edge of the tub. Scoop a handful of fish with a small net into a transparent container with a layer of water, place a ruler or a sheet of millimeter paper underneath, and read the length from snout to base of the tail. If you prefer photographing, place the ruler next to the container in the frame. Measure more than one fish: the spread within a group says more than a single outlier.

The rule is a rule of thumb, not a law. In the north of the Netherlands, where the first frost often comes earlier and lasts longer, you can comfortably set the threshold at 2 cm. In a shallow tub of 30 cm the water cools faster than in a one-metre tub, and then every millimetre counts. For the winter of 2026 to 2027, the same consideration applies as every year: measure at the end of September, not only in November.

Overwintering young fish: what the fat reserves do

Fat is the fuel with which a fish gets through winter. Between November and March a medaka eats almost nothing, but those months still have to be bridged. A fish hatched in September or October simply has had less time to build up that store than a companion from May. Whether overwintering young fish succeeds therefore depends on what it builds up in September.

That's why the 1.5 cm rule works so well: length is not a goal in itself, but a reliable indicator of how much energy a fish carries. If you want to boost that supply before the cold sets in, these are the dials to turn:

1. Feed more often, three to four small portions a day instead of one large one.
2. Choose higher-protein and higher-fat food in the last weeks before the cold, not the maintenance food of summer.
3. Measure the water temperature daily, so you know when growth truly stalls.
4. Keep the stocking density low, because in a crowded tub the smallest always gets the least.
5. Give the tub a sheltered spot with afternoon sun, so the water stays above 15 °C for as long as possible.

Indoors or outdoors: two routes through the cold months

There are two routes for tackling late fry overwintering: inside or outside. If your late fry does not make the limit, you set up an indoor arrangement. An aquarium or a large bucket at a stable 18 to 22 °C is enough. The fish keep eating and growing, without the blow of falling outdoor temperatures. It costs electricity and space, but you do keep every fish you have in October.

If the juveniles are big enough, overwintering outdoors has advantages. It takes no space in the house and the fish go through a real winter dormancy. The condition is depth: at least 60 cm of water, so that enough liquid oxygenated water remains under the ice. Keep an opening in the ice with a float or an air stone, and never let the ice freeze over completely.

How to get an undersized medaka through winter

If in doubt, choose the safe side. With overwintering young fish that is almost always the aquarium side. Putting a fish indoors costs a bucket and a power outlet; a wrong guess costs you the whole spawn. If you have room, bring in all borderline cases and leave outside only the fish that are well over 1.5 cm.

• Measure in the last week of September, not only after the first night frost.
• Note per tub how many fish stay below the threshold, so you know how much space you need and can compare in 2027 whether you should start feeding earlier.
• Acclimatise the fish to warmth in two steps, with an hour or so between each step.
• Check in February whether the outdoor overwinterers still swim clearly and have no white spots or fungus.
• Only put an outdoor tub back in the sun once the water rises above 12 °C again in March.

For the species itself, the description of the Japanese rice fish and the species page at FishBase are a good starting point; they describe the animal as an inhabitant of shallow, still water in rice paddies and marshes. For the frost forecast, check the KNMI, because the date on which it starts freezing in your region determines how strictly you should hold to the threshold.

Further reading

• Removing sludge before winter: this is your last chance
• Making the pond autumn-ready: leaf litter, sludge and ammonia
• How to recognise that your medaka are ready to spawn

Frequently asked questions about juveniles and winter (FAQ)

What happens if a fish under 1.5 cm stays outdoors anyway?

The chance of death becomes high. Such a fish has too little fat to bridge the months without food, and the first cold night with ice formation is then often the end.

Can I still do anything in September for a laggard?

Yes, within limits. Feeding more and more often, and higher-protein food, helps, but a fish that hatched in August won't make up months of growth in four weeks.

Does the 1.5 cm rule apply to every medaka variety?

For the common varieties it works well. For finer or weaker lines, and in regions with harsh winters, it's better to set the threshold at 2 cm.

How warm should the indoor tank be?

Between 18 and 22 °C a young medaka keeps eating and growing. Below 15 °C that stops, and then an indoor tank gains you nothing.

Can a fish still be brought indoors later in winter?

It can, but every jump in temperature costs energy. If you bring it in during January, do it gradually over a day or two and accept that growth only really picks up in March.