Preparing Medaka for Winter
8/12/20268 min read
Preparing Medaka for Winter
A Japanese-inspired guide to overwintering Medaka outdoors in the UK
Medaka are remarkably hardy fish and, when properly prepared, healthy adult Medaka can successfully overwinter outdoors.
In Japan, outdoor Medaka are commonly left to follow the natural seasonal cycle. As temperatures fall, their metabolism slows, feeding reduces and eventually they enter a period of winter dormancy.
The key to successful overwintering is preparation before the cold weather arrives rather than trying to intervene once the fish have already become dormant.
For UK keepers, winter can bring rapid changes in temperature, frost, snow, wind and prolonged periods of cold weather. A little preparation can make a significant difference.
🍂 Prepare Before Temperatures Fall
Begin preparing your tubs while your Medaka are still active and feeding well.
Healthy fish with good reserves are much better equipped to cope with winter than fish that enter the cold season already stressed or underconditioned.
Before winter:
Check that all fish are healthy and free from obvious illness or injury.
Remove excessive dead or deteriorating plant material.
Remove anything from the tub that is likely to decay heavily during winter.
Check that the tub is stable and cannot easily be blown over or damaged.
Ensure there is sufficient water depth for the fish to retreat into deeper, more stable water.
Avoid making unnecessary changes once the fish have entered winter dormancy.
Keep an eye on weather forecasts so that you can prepare for sudden cold snaps.
🌡️ Let the Fish Follow the Seasons
Medaka are ectothermic, meaning their body temperature and metabolism are governed largely by their surrounding water temperature.
As the water becomes colder, their activity naturally decreases.
You may notice that your Medaka:
Become less active.
Spend increasing amounts of time resting near the bottom.
Show considerably less interest in food.
Eventually become almost completely still.
This is normal winter behaviour.
At around 15°C and below, appetite and activity will begin to decline. As temperatures approach 10°C and below, feeding should be greatly reduced and eventually stopped.
At very low temperatures, Medaka can enter a genuine winter dormant state. Their digestive system slows dramatically, so feeding at this stage can do more harm than good.
Do not feed simply because you think the fish look hungry. Follow the water temperature and their behaviour.
🐟 Reduce Feeding Gradually
As autumn progresses, begin reducing the amount of food offered.
There is no advantage in continuing to feed a summer diet when the fish's metabolism is slowing.
A sensible seasonal approach is:
Above 15°C
Continue feeding, but begin adjusting the quantity as activity decreases.
Around 10–15°C
Reduce feeding considerably. Small amounts are preferable to large meals.
Around 10°C and below
Stop feeding when the fish are no longer actively feeding.
During winter dormancy
Do not feed.
Uneaten food can quickly become a water-quality problem, while food that is poorly digested can cause problems for fish whose digestive system has slowed considerably.
A lower-protein, easily digestible seasonal food can also be useful as temperatures begin to fall.
🍁 Add Leaves for Natural Winter Cover
One of the things I particularly like about the Japanese approach to overwintering Medaka is the use of dried leaves.
Persimmon leaves (柿の葉 – kaki no ha) are traditionally used by Japanese Medaka keepers and make an excellent addition to an outdoor winter tub.
As they slowly break down, the leaves provide:
Natural hiding places.
Additional surfaces for beneficial microorganisms and biofilm.
Shelter for resting Medaka.
A more natural winter environment.
Gentle tannins released into the water as they decompose.
Indian almond leaves (Catappa) can also be used and provide similar benefits.
Leaves should be clean, completely dry and free from pesticides or other chemical treatments. There is no need to boil good-quality dried leaves; if you wish, they can simply be rinsed before use.
Do not add huge quantities. The aim is to provide natural cover rather than fill the tub with decomposing organic matter.
🌿 Remove Plants That May Deteriorate
Not every pond plant should be left in the tub throughout winter.
Plants that die back and rot heavily can contribute to deteriorating water quality and consume oxygen as organic matter decomposes.
Before winter, look carefully at your plants and remove:
Large amounts of dead foliage.
Plants that are already deteriorating.
Excessive floating vegetation.
Anything that is likely to completely decay underwater.
Leave suitable hardy plants where appropriate, particularly those that provide useful cover.
Think about the condition of the whole biotope, rather than simply leaving everything in place because it is natural.
🪣 Choose a Stable Winter Environment
Water volume and insulation are important during winter.
Larger volumes of water generally experience slower temperature changes than very small containers.
Where possible:
Maintain a good depth of water.
Avoid extremely shallow winter containers.
Position tubs somewhere sheltered from strong winds.
A wall, fence or sheltered corner can provide useful protection.
Avoid locations exposed to constant winter winds.
Raise containers slightly from the ground where practical to reduce heat loss and improve stability.
Polystyrene or insulated containers can be particularly useful for smaller setups.
For existing outdoor tubs, insulating the outside of the container with suitable materials can help reduce rapid temperature fluctuations.
The aim is not to keep the water warm.
The aim is to slow down sudden changes.
❄️ What If the Water Freezes?
A thin layer of surface ice is not necessarily a disaster.
The important distinction is between the surface freezing and the entire container freezing solid.
If the surface freezes, the deeper water beneath the ice can remain liquid and provide a refuge for the fish.
Never smash the ice.
Hitting or breaking the ice can send shock waves through the water and disturb Medaka resting beneath the surface.
Do not pour boiling or hot water onto the ice either. A sudden temperature change can be extremely stressful.
Instead, allow the ice to thaw naturally whenever possible.
If your area is experiencing prolonged severe freezing conditions, take additional precautions to prevent the entire container from freezing.
🌬️ Don't Seal the Tub Completely
Winter protection does not mean creating an airtight container.
Medaka still require gas exchange with the atmosphere.
If you use a cover to protect a tub from snow, leaves or extreme weather, ensure that there is still adequate ventilation.
A completely sealed container can lead to oxygen and gas-exchange problems.
🚫 Avoid Disturbing Dormant Medaka
One of the biggest mistakes during winter is trying to "help" the fish too much.
Once Medaka have become dormant:
Don't stir the bottom of the tub.
Don't repeatedly move the container.
Don't carry out unnecessary large water changes.
Don't net the fish unless absolutely necessary.
Don't disturb them simply to check that they are alive.
Don't suddenly move them into a warm indoor environment.
During winter, stability is your friend.
The fish may appear almost completely motionless, but this can be perfectly normal.
Absolutely — I’d add this as a dedicated section, because it’s an important part of winter preparation and fits the Japanese approach of allowing dormant Medaka to remain as undisturbed as possible.
💨 Reduce Water Movement
As Medaka become dormant, their activity and metabolism slow considerably. During this period, the aim is to create a calm, stable environment where the fish can rest without unnecessary disturbance.
Consider turning off your air bubbler during the coldest part of winter and/or reducing your filter or pump to its lowest suitable setting.
The aim is to have very little water movement.
Strong bubbling, currents and circulation can continually disturb resting Medaka and encourage them to remain active when they would naturally be conserving energy.
Every movement requires energy. During winter, when food intake is greatly reduced or stopped, unnecessary activity can deplete the fish's limited energy reserves. Prolonged stress and energy expenditure may leave fish in poorer condition and potentially more susceptible to illness.
A calmer winter environment
Turn off air stones or bubblers once the fish have entered winter dormancy, where appropriate.
Alternatively, reduce aeration to the minimum required for your particular setup.
Reduce filters or pumps to their lowest practical setting.
Avoid strong currents across the tub.
Keep the water as calm as possible without compromising water quality or gas exchange.
Avoid repeatedly switching equipment on and off simply to check the fish.
This is particularly relevant to small, shallow tubs and heavily filtered systems, where even a relatively small pump can create considerable movement.
The objective is not to eliminate oxygenation or filtration at all costs. Every setup is different, and heavily stocked systems may still require some circulation. Instead, aim for the lowest level of movement that maintains a healthy environment.
Think of winter as a time for your Medaka to conserve energy rather than spend it.
Calm water, minimal disturbance and stable conditions are the goal.
💧 Water Changes During Winter
Avoid large water changes when Medaka are dormant.
A large water change can introduce water with a significantly different temperature and chemistry, creating unnecessary stress.
If an emergency water change is genuinely required, keep the temperature and water parameters as close as possible to the existing water.
Otherwise, allow the mature outdoor biotope to remain relatively undisturbed.
🧬 Not Every Medaka Is Equally Winter Hardy
This is particularly important when purchasing specialist or newly imported varieties.
Although Medaka as a species are extremely hardy, individual strains and breeding lines can differ in their ability to tolerate winter conditions.
Wild-type and long-established hardy lines may cope very well with outdoor winters, while some highly selected ornamental varieties may be less robust.
If you have obtained a new variety, particularly from imported eggs, do not automatically assume that it has the same winter hardiness as established Japanese outdoor lines.
Ask the breeder:
Where did the parent fish originate?
Have the parents been kept outdoors?
Have previous generations successfully overwintered outdoors?
What temperatures have they experienced?
Are they first-generation imported fish or an established UK line?
First-generation fish from imported eggs should not automatically be considered winter hardy.
Give newly acquired fish time to acclimatise and establish themselves before expecting them to cope with a UK winter.
🌸 When Spring Arrives
Spring can actually be one of the most dangerous periods for overwintered Medaka.
The days may become warm while nights remain extremely cold, resulting in large temperature fluctuations.
Don't rush the transition.
When water temperatures begin consistently rising above around 10°C, watch your fish closely.
As they become active again:
Offer very small amounts of food.
Only feed when they are actively interested.
Gradually increase feeding as activity and appetite return.
Remove any uneaten food.
Continue monitoring nighttime temperatures.
Avoid suddenly moving fish into a heated environment.
Water quality should also be assessed after winter.
Rather than immediately performing a huge spring clean, make gradual changes. Small, controlled water changes and gentle removal of accumulated debris are preferable to completely stripping down a mature biotope.
As temperatures stabilise, plants can be tidied, the tub can be prepared for the breeding season and normal feeding can gradually resume.
🇯🇵 The Japanese Approach: Prepare, Then Leave Them Be
Perhaps the most important lesson from Japanese outdoor Medaka keeping is that winter is not a time for constant intervention.
Prepare the tub properly in autumn.
Provide shelter.
Reduce feeding as temperatures fall.
Protect against extreme weather.
Then allow the Medaka to follow their natural seasonal cycle.
Healthy fish + stable water + good preparation + minimal disturbance = a much better chance of successful overwintering.
Every winter is different, and the conditions experienced in Japan cannot simply be copied directly to the UK. Your local minimum temperatures, container size, water depth, exposure and the particular Medaka strain all need to be considered.
When in doubt, speak to the breeder about the winter history of your particular line.
The Medaka Emporium Winter Principle
Don't try to keep winter away from your Medaka — prepare them properly so they can experience it safely.
The goal of overwintering is not to keep Medaka warm and active throughout winter.
It is to allow healthy fish to slow down naturally, remain protected and stable, and emerge in spring ready for another breeding season.
