Winterizing a Koi pond and pond fish

Putting koi through a typical UK winter experience is regarded by many as a matter of potluck as to whether they will pull through to the spring. That is, we keep our fingers crossed and hope that the koi have eaten sufficiently well and that the ice doesn’t come too fast and thick that the koi will somehow manage and still be there for us in the spring. Well to some extent, we are at the mercy of the elements and some aspects are out of our control.

But if we stand back from the ‘unknown’ that is almost upon us, we can analyse some of the factors affecting the fate of our koi’s health over winter. These can be broken down into different variables, many of which we can actually have a positive influence on and thereby remove a large degree of ‘luck’ from overwintering koi.

Koi have been over-wintering and surviving frozen ponds for many hundreds of years and our climate in the UK can be a little more extreme than Japan and our ponds alien and unnatural compared to their natural mud lake existence. However, as long as we are aware of these differences we can accommodate our koi’s demands accordingly.

If all goes to plan, you should be able to rest in the fact you have acted as a responsible koi keeper that in your preparations for the winter, your fish are in good hands and that they will emerge in spring in good condition, even if your millennium party has been as long and as loud as you are planning it to be!

What happens to a koi pond in winter? Winter can be regarded as a test of stamina just as a long distance walk would be for us. Providing your preparation and attention to detail has been adequate in the preceding period then the koi’s journey, although not easy will be well within the grasp of a well-prepared koi and koi pond. Perhaps quite surprisingly, the threat from disease during the winter is not a realistic one with colder water reducing growth and infection rates of pathogenic bacteria. In addition, reduced feeding and metabolic rates of the ‘hibernating koi’ means that ammonia production is also negligible making water quality a reduced concern. Studies suggest that over-wintering pondfish have a metabolism of no more than 5% that of normal summer rates. The colder water also has a greater capacity to hold DO (dissolved oxygen) and as the oxygen consuming organisms (bacteria, protozoa etc.) are metabolising at a reduced rate, their need for oxygen is also reduced.

Also a strange thing happens to water when it cools. The density of a material usually increases as it gets colder and this is true for water, but only down to 4 degrees C. Between 4 degrees C and freezing, the water actually gets less dense and floats above the denser 4-degree water, eventually to form ice if the air temperature drops below freezing. Koi and other pondfish take full advantage of this phenomenon by sitting on the pond bottom. In very still and deeper water quite a stable boundary can form between the less dense 0-4 degrees C water and the denser warmer water.

This barrier is called a thermocline and the phenomenon of different layers forming in water is called stratification.

A thermocline is not likely to form in most koi ponds due to their relatively shallow nature and the mixing action of pumps and filters. Even so, any pockets of warmer water in this dynamic situation will still have a tendency to sink providing a warmer retreat for the koi. If this strange phenomenon did not occur then all aquatic life would freeze to death on the bottom of ponds each winter.

Heat and Light In many other respects a koi’s physiology is in tune with its environment, picking up information about its environment and responding to it. Two key stimuli for koi are heat and light.


Koi are poikilothermic, which means that they take their body temperature from that of the surrounding environment. When it is warm, they are active, feeding, growing and producing the waste to match. The reverse is true as the water temperature drops. As long as the drop in water temperature is gradual as it usually is between summer and autumn, lagging slightly behind the reducing air temperature, then koi should adapt to what for them is a quite natural phenomenon. They have no choice in the matter and respond to cooling water by a drop in activity as their metabolism slows down.

Photoperiod (daylength) also plays a vital role in preparing koi for the winter. The photoperiod interacts with koi to control the secretion of a number of key hormones, including a growth hormone. The reduction in daylength into autumn prepares the physiology of the koi to ‘switch off’ as winter approaches. The interaction of heat and light is effective at preparing koi for their winter break. We can take some comfort in this because as a result, our koi are likely to have been preparing for winter earlier than we have.

In their natural environment, koi and particularly carp have a number of benefits over our pond-kept-koi. Firstly, stocking rates are likely to be less with knock-on effects for better water quality. Natural water bodies being deeper and larger will also hold their temperature better, providing a more stable environment than most koi ponds, yet ice may well completely cover the surface. Another key feature of natural lakes or ponds is the soft and silty sediment into which fish can settle, affording further insulation over the winter period. A real problem in koi ponds where the substrate can be a bare hygienic liner or fibreglass membrane is that by rocking around on the flat surface throughout the winter months, koi can develop sores which may be sites for further infection even leading to ulceration.

The carp’s physiology is geared to over-wintering and this physiology has largely been inherited by koi. If this were not the case, then a shortening photoperiod would not have such a profound effect on the entire physiology of the fish, preparing it to ‘shut down’ for a ‘fallow period’.

A cold period is also required to re-set a koi’s biological clock and the process of coming out of a cold period with increasing daylength stimulates the physiology of koi to prepare to breed. However winter can be a risky period, particularly in those koi ponds where the recommended minimum actions have not been carried out. Having said this though, I have heard of koi keepers in Finland over-wintering their koi at -20 degrees C successfully for 15 years. This is a testimony to their understanding of koi and their overwintering needs.

Conversely, many Japanese koi farmers actually opt to bring their stock inside for winter if they have the facility to do so, bearing in mind that their livelihood depends on them successfully rearing and selling sufficient quantity of fish. Bringing them in for their short sharp winter reduces certain risks that large numbers of juvenile koi may experience in natural ponds and lakes. However, such actions also expose those fish to other risks that fish in ponds would not experience over winter, such as a continued threat of disease in the warmer and more densely stocked tanks.

There are arguments for maintaining koi at the same temperature all year round and I have seen koi kept in the tropics quite satisfactorily where there is no option but to keep them at a similar temperature 365 days a year. However, the daylength also remains pretty constant in such areas unlike the UK and Europe where the decreasing photoperiod in the autumn prepares koi for the oncoming winter.

I have tried to rear juvenile koi from their first summer right through winter in warm water ponds. In ponds which actually steamed in the winter, the koi went through the motions of feeding quite happily but did not grow significantly, a factor I believe of the short daylength in winter interfering with their secretion of growth hormone. That is, the temperature was telling them to feed and be active but the photoperiod was telling them to ‘hibernate’.

This could also be true for koi in the UK and Europe kept in heated conditions over winter. They may feed, but do they really grow as much as the equivalent temperature in summer?

Those koi keepers who do heat their ponds over winter do gain the benefit of seeing their fish feeding and gaining 365 days enjoyment from their fish.

Some koi keepers who heat also do so of course to prevent their koi from experiencing such a risky winter period. However, there is a cheaper alternative that does not include the expense of heating and success can be achieved economically by insulating a pond. In this way the koi experience the helpful influences of a winter, but are not exposed to the risks associated with a full-blown, fully exposed winter.

Bubble-wrap option

This is one of the most practical, economical and reliable ways of preventing your koi from being exposed to the risks of a full-blown winter. A simple frame or hooped arch can be constructed from over-flow pipe and push-fit connections onto which a layer of bubble-wrap can be attached. Heavy-duty bubble-wrap costs about 1.50 per square metre and can be used for several years depending on how well it is attached to the hoop structure. Its life will be shortened if wind is allowed to get inside the ‘tunnel’ and blow the sides out. This form of insulation can be used in conjunction with a small 100w pool heater, installed through a greenhouse-type thermostat which can be set to come on during frosty nights.

Although its not the most aesthetic protection, it can easily be dismantled and re-used each year and has a proven track record of protecting fish and ponds from the harshest of winters.

The minimum all koi keepers should do is to keep ice from fully covering the pond, and this can be achieved by using a 100w pond heater.

In winter with no food entering the pond, ammonia and nitrites are not likely to reach toxic levels. DO is not an issue either as the metabolism of those organisms consuming oxygen will de depressed. However, pH can be a cause for concern and should be monitored, particularly in the early spring during the thaw.

Rainwater and thawing snow etc. being a relatively pure form of water has no buffer present and in fact will be slightly acidic, having dissolved CO2 on its way down from the heavens. Although this free water change makes the pond water very clear, perhaps the clearest you are likely to see it, it also comes with the threat of an undesirable drop in pH. This can be countered quite simply by adding a mesh bag of limestone or crushed shell in the filter. It is good practice for this to be installed permanently in the filter as it guards against wild swings in pH all year round and will maintain a healthy, slightly alkaline pH.

Bringing koi indoors.

This is another option for over-wintering your fish safely, but it too has a number of consequences. The main problem is space. If the koi are likely to be more cramped indoors than they are in the garden pond then it is better to leave them where they are with adequate over-wintering protection. There may be some value in bringing a number of your smaller fish inside, as they are likely to be most at risk over winter. A large well-filtered aquarium or tank would suffice.


The key issue about over-wintering a pond and its inhabitants safely is stability. The stability of water temperature and water quality is essential, ensuring that any changes that do occur are gradual and within the natural limits of the koi.

This is quite easily achieved in deeper ponds that are deeper and larger in volume but can be a problem to all koi keepers whatever their set-up either side of winter when they are subjected to isolated warm spells in September, October, February and March.

Indian Summer?

Experience shows that if koi show some interest in feeding during these months and take some food, there is significant risk involved for the koi. If you suspect that the unseasonable warm weather is only likely to be an isolated few days as it is during these months then koi, not being able to look into the future may have food in their guts for a long time. Although feeding would be common behaviour in wild carp, feeding koi just prior to a cold snap can offer a number of undesirable yet avoidable risks for the fish.

Food that sits undigested in a gut for a long time can putrefy and lead to other complications. However, considering how many koi and other pondfish are likely to have been fed during such isolated warm spells over the years, I am surprised that more pondfish have not been lost over the winter in ponds across the country. Is it really such a lethal practice? If your koi have had a good summer’s feeding then they will not need extra food during such isolated warm periods. To feed or not to feed? The choice is yours!

Disease You are unlikely to witness diseased or unhealthy fish during cold and icebound conditions but any problems that do occur are more likely to manifest themselves in the spring when water temperatures and disease populations can explode ahead of the koi’s ability to fight them. Dose a pond with a series of broad-spectrum antibacterial treatments in late February to decrease the numbers of pathogenic bacteria.

How to keep pond conditions stable over winter

Several straightforward procedures can be carried out to improve the stability of water conditions over winter. Filtration should be maintained, even at a reduced turnover. Care should be taken with any exposed pipework as frozen blockages can easily occur, especially if the turnover rate is reduced over winter.

Pumps should be raised off the pond bottom to minimise the mixing of deeper warmer water and improve the temperature stability of the deeper water. The same is true for aeration which can be turned off as extra aeration is not required at these temperatures.

Leaves should be prevented from falling into the pond as their decomposition will lead to a rise in ammonia and drop in dissolved oxygen. The leaves from certain trees and plants also produce toxic by-products when they decompose.

How to heat your pond.

If you feel heating is an option, then there are several alternatives to choose from.


Gas-fired boilers are the top of the range pond heaters. There are two options, either a feed off a home central heating system or an independently installed adapted swimming pool boiler system.

An advantage of using a gas-fired heater is that once installed, it is relatively cheap to run and has the power to maintain a suitable temperature quite comfortably. Difficulties can arise when using a feed from the house system with timings to coincide with domestic use and care must be taken that any exposed surfaces in the boiler or heat exchanger are inert. Exposed copper surfaces must be avoided at all costs and stainless steel heat exchangers are ideal. Swimming pool boilers can cost up to 1000 for a koi pond, with the installation on top of that.

Pond Covers

The best type of cover is heavy-duty horticultural bubble wrap mounted on a hooped frame above the water. Overflow pipe for the frame and bubble-wrap for a 3mx10m pond would cost approx. 200.

Advantages are that it can be disassembled and re-used each year and there are no running costs. However, it is very unsightly and if not well anchored could end up in next door’s garden!

Indoor Ponds

Heating is not required in such ponds, where they may be installed in a built-on outhouse or conservatory. They allow year-round viewing and feeding but are expensive to build and condensation can be a real problem.

Pool Heater (100w)

These are not designed to heat the pond water but prevent ice from totally covering the pond. They cost 25 + cable + thermostat. They are cheap and easy to install. They are small, easily hidden and cheap to run. However, unless fitted with a thermostat, are on all the time or rely on you predicting a frozen spell. It does not protect a pond from the full extremes of winter.


The thermometer is probably the cheapest yet most useful piece of over-wintering kit. It gives us valuable information as to what action to take be it heat or not to heat or an indication of what the koi will be going through and whether spring is on the way or not!

Do + Don’t


Prevent ice from fully covering the pond

Service and clean the filter (settlement chambers etc.) and pond before over wintering.

Cover the pond with a fine mesh net to stop leaves entering the pond and settling on the bottom.

Treat the pond with a general anti-bacterial treatment in the first warm days of activity in spring.

Give the fish one last visual check before winter and actively examine the underneath of fish in spring for sores.


Panic. Provided you have fed your koi well over the summer, they will have reserves to overwinter. Research has shown that koi can live for 200 days without food over winter before encountering real problems.

Smash ice if it forms. Use a hot saucepan or kettle and keep it ice-free with a 100w pond heater.

Leave aeration on as it will mix all water depths causing the water to chill quicker.

Feed in isolated sunny spells, even when encouraged to by koi activity. Particularly resist in months of November to February, as prolonged cold periods will almost certainly return during these months.


Put a small pond heater on a thermostatic control to come on during a frost. Fit it and forget it.

Choose a black fine mesh net to prevent leaves falling into the pond. Not as visually obtrusive as a green net.

Keep a check on pH as it can nose-dive during a thaw. Use cockleshell or limestone to counter large pH swings.

Go to aquatic centres in Autumn. There are likely to be lots of bargains and savings on offer during end of season sales etc.

When Ice Strikes

If ice does catch you unawares and totally covers your pond, then thaw it out with hot water. A floating football is not always effective at leaving a hole in the ice as I have seen them lifted out of the pond by ice.

If some fish appear to be affected during the winter, it is likely that it is a problem overrunning from the summer as pathogenic problems progress so slowly at colder temperatures that there is little danger of new problems arising during winter. If possible, isolate the affected fish indoors by bringing some pond water inside to an indoor tank or vat and gently raising it to see if the fish’s behaviour improves. Remember, having removed a fish in winter it is unwise to return it to the pond before springtime.

Temperature graph

These temperatures were recorded at the pond bottom. They show that even in a small, shallow pond (4′ deep) that the water at the pond bottom is warmer than that at the surface. Through the winter, there was frequently an ice covering the pond and yet temperatures at the pond bottom hovered around 5 degrees C. This also shows how aeration or pumping water from the bottom over winter can be hazardous for the fish.

Notice also how the photoperiod changes throughout the year. The photoperiod and water temperature act in tandem to ‘inform’ the koi that winter is approaching.

Winter Pond Advice.

All you need to know for your pond and pond fish to survive the harshest winter in a garden pond.

Kill blanketweed and string algae.