top of page

How to Prepare Your Pond for Winter

As the temperature lowers and leaves fall from the trees, our attention at Keruto Koi turns to the coming winter months. Winter can be a tough time for many animals and koi are no exception. But what can we do, as koi keepers, to give our fish the best chances of surviving the tough winter months?

What happens to koi in the winter?

It is first important to consider exactly what happens to koi during the colder temperatures. Koi are poikilothermic animals which just means that they are cold-blooded and that their body temperature is regulated by the water they are in. Because of this, their body functions are dependant on the temperature of the water.

You may have noticed that in summer, and especially on hot days, your fish will be a lot more active, swimming a lot more and even jumping out of the water like the ones here:

The opposite happens in colder weather. As the temperature decreases, the activity level of the fish lowers. Koi will slow all their movements down and will often prefer to hide rather than swim in the open. Their metabolism will also reduce and therefore they will have less of an appetite and will refuse most food.

When the temperature drops to about 10°C, the koi will go into a state of torpor. Torpor is a type of hibernation where the fish will reduce all of their bodily functions to save energy. Torpor is often a shorter form of hibernation but everything else is very similar – koi will have a lower body temperature, reduced heart and breathing rates, slower reaction times and metabolism and a reduction in primary body functions. However, torpor allows the fish to react to the temperature of the water. If the water is below 7°C, the fish will just sit on the bottom of the pond, where is it warmest, and appear to be asleep. However, if the temperature is between 7°C and 10°C, you will likely see some activity from the fish, they will slowly swim around, and you may even see them coming to the top of the pond if the sun is out!

So, what should you do to prepare for colder weather? First note that all of your preparations should be done during November time as this is when your fish will still be active enough that any preparations will not affect them, but you do not want to do your preparations so early that you will then need to redo anything before winter.

Cleaning and maintenance

The run-up to winter is the best time of year to do a big annual clean of your koi pond. We recommend that you use this time to maintain and trim your plants and clean out your whole pond including any build-up of pond sludge on the base of your pond. For more information, have a look through our blog post ‘Essential Pond Maintenance for Autumn’ which can be found here:

Predators during colder months

Many of a koi’s typical predators will not hibernate over winter which will leave them hunting for food while your koi are in a state of torpor and therefore very vulnerable. Especially on the coldest days, your koi will likely group together at the bottom of the pond where they will find the warm spots. Since they are all in a group and have very little energy to move, they will be prime targets for predators. It is a good idea to have protection against such predators all year round but the end of autumn, when you are preparing for winter, is the best time of year to check any protection and upgrade or reinforce if necessary.

Feeding before and during winter

As mentioned above, when koi are in a state of torpor, their metabolism will slow down and their ability to digest food will be greatly reduced. If you continue to feed your koi as normal during this time, they will either refuse the food in which case, you risk polluting the water with uneaten food or they will end up with undigested food in their bodies which is often fatal if there are large amounts of food.

Some of our larger koi eating heartily to prepare for torpor.

We recommend that for the end of autumn and the beginning of winter, you should swap your koi over to a wheat germ-based food. These particular foods will contain more fats, vitamins and slow-release carbohydrates and therefore will help your koi build up their fat stores ready for torpor.

As the temperature decreases, they will be looking for less food so be sure to also decrease the amount of food you are giving them. When the temperature drops to 10°C or lower, you should stop feeding your fish entirely until winter ends and the temperature rises again. You may find that your koi will come to the surface looking for food on warmer days and this is a great time to offer them a little bit more of the wheat germ food. Take care to not give them too much, it is better to leave them a little hungry than to overfeed them at this time of year, but small quantities of wheat germ on nice days leaves them a little healthier come spring.

Salting your pond

Koi keepers often add pond salt to their ponds for many reasons. Salt can kill most of the common parasites in pond water, it helps to reduce the toxicity of nitrites, it can help to boost your koi’s natural defences to disease and salt adds beneficial electrolytes to your water which helps both your fish and plants.

Koi also produce their own natural salt to give them these benefits but when they are in torpor, this salt production is one of the bodily functions that will slow down over winter. Therefore, in order to keep your koi as healthy as possible, it is best to increase the concentration of salt over the winter months. During most of the year, your koi pond should have about 1% salt but to compensate for the loss in natural salt, we recommend increasing this to around 3% salt for November-January.

What should be turned off for winter?

Firstly, if you have a waterfall, that definitely should be turned off! Any water travelling over the waterfall will be exposed to colder air temperatures and will cool down, therefore chilling the pond water further. Also, if temperatures are low enough, there is a risk that the water falling down the waterfall could freeze leaving dangerous icicles hanging above your koi pond. If these icicles were to fall, they could damage or even kill your koi!

A Nexus 320 filter like the ones here at Keruto Koi that run all year round

It is a common myth that you should turn both your pond pumps and filters off during the winter months. This is absolutely not true, and, in fact, there are many reasons to keep both of these on during the winter. For more information, keep an eye out for our future blog posts about pond maintenance.

The final piece of pond equipment to consider is a UV steriliser. If you have any of these, we recommend that you disconnect these from the rest of your system and drain them of all the water. They should then be stored in a warm and dry place for the winter. If a UV steriliser is left running and the water inside freezes, this can destroy the equipment. Even a light frost can permanently damage a UV steriliser and make it dangerous to use with your pond.

What if my pond freezes over?

A frozen-over pond can be very dangerous to koi due to a build-up of trapped carbon dioxide and other harmful gases under the ice, so the final thing you will want to consider in your preparations is whether you will cover the pond in any way over the winter. This is not essential in the UK but can be very helpful in keeping the temperature of your pond a little higher. If you are in an area where you can expect the surface of your pond to freeze over (more so in the northern UK rather than the southern UK), a pond cover should prevent your pond from freezing over which will make life in the pond a lot more comfortable for your koi.

The best pond cover you can use to prevent the risk of your pond freezing over is a greenhouse or poly-tunnel purpose fit to your pond. Obviously, this can be very expensive and out of a lot of people’s budgets, but a well-fitting lid should do the job. In fact, one of the best alternatives to a greenhouse is a solar cover for a swimming pool. These covers are designed to allow the sun’s rays through the cover to warm the pond water but then trap the heat so that heat loss is minimal. This results in the pond water staying above freezing even overnight. This is not guaranteed, however, and in especially cold weather, other measures should be considered to prevent freezing.

Many of the ponds at Keruto Koi are in a custom-built koi shed to prevent freezing and to stop predators from accessing the koi.

Instead of covering the pond, you could use a pond heater to keep the temperature of the water above freezing but this could potentially be an expensive alternative as the costs for running some pond heaters can be high. Also, if this is your only method of preventing ice from forming, we recommend that you have a second heater to act as a backup in the case that the first breaks or stops working.

Top Tip: Leave a tennis ball floating in your pond and, if your pond does freeze over, the gentle movements of the tennis ball will break up the ice as it forms. This will allow the carbon dioxide to escape the water and also allow oxygen to enter the water which is vital to your koi’s survival over winter!

Just popping a tennis ball in your pond is a simple way to prevent the water from freezing over!

In the case that your pond does freeze over, don’t panic! Firstly, you need to ensure that there is a hole in the ice to allow the exchange of gases. The quickest and safest way to do this is to use a small hot water bottle. You should lay it on the surface of the ice and allow it to slowly melt through until it drops into the water.

Never, ever break the ice on the surface of the water as this will send shockwaves throughout the pond, potentially injuring and even killing your fish. Once you have an opening in the pond, it is best to just leave the pond alone to avoid stressing the fish but be sure to check regularly that the hole is still there.

As long as you prepare for winter and ensure that you know what to do when the temperatures drop, there is no reason why your koi should not come out of winter happy and healthy and ready for spring.

For more information about how to look after your koi carp, keep an eye out for more blogs in our ‘Pond Maintenance’ series which can be found here:

998 views0 comments

Recent Posts

See All


bottom of page