Koi pond winter management. Frozen ponds and freezing weather.

The vast majority of us don’t heat out ponds. Whether this is because we can’t afford to, don’t feel our fish require it or we just haven’t got around to it yet – what this means is that our koi will be exposed to what our winter has to throw at them.

I believe that koi benefit from a winter and will thrive and breed in the following summer all the more for experiencing a winter period. We do however have a duty of care to ensure our koi are able to withstand what our winter throws at them in our artificial man-made ponds.

True, global warming does appear to be extending our koi’s active phases, but there is still the high probability that our koi will experience a harsh winter. In fact, the Met Office is predicting an unusually harsh winter this year.

Winter Extremes

The three extremes we need to be wary of for our koi are:

1. Extreme and rapid drops in temperature

2. Extremely cold winter weather

3. Extremely lengthy period of very cold weather.

A carp’s natural temperature range in the Caspian Sea is -10C to 28C. Within this massive freshwater body, they do however have the benefit of the deep waters in which they can take refuge.

Of course, our own ponds are not as large nor as deep as the Caspian Sea and so our koi are likely to be at a greater risk from the extremes of our winter weather.

Winter extremes can harm our koi in several ways

a. Rapid drops in temperature  - Shocks to the system. As I write this, we are expecting a 21C warm spell in the last days of October. This could very easily be followed by long days and nights of sub-zero temperatures. Such extreme changes are not natural for our koi (even though the temperatures themselves are tolerable). This will result in stress and an increased susceptibility to disease (should any opportunistic pathogens actually be present at these temperatures).

b. Icy water. If your pond’s depth is not sufficient (4’ deep minimum to be sure) to form a barrier between your fish at the bottom of the pond and the icy air, then your fish will experience colder water than is naturally tolerable. This is likely to lead to a reduction in blood circulation in exposed tissues – especially fins and other bodily extremities, leading to an increased risk of tissue necrosis and subsequent bacterial and fungal infection in the spring.

c. Lengthy periods of cold weather. In the Caspian Sea, carp aren’t subjected to such cold temperatures as they might experience in a shallow koi pond, and certainly not for months on end. In their native environment, they are free to migrate to deeper and warmer areas of the water. Again, this unnatural (and unreasonable) demand on their physiology is likely to manifest itself in stress-related problems in the spring when the pond, the koi’s metabolism and pathogens start to warm up.

Short term weather forecast.

You can prepare for cold snaps (and winter itself) by insulating the surface of your pond with a frame of bubble wrap. This is a quick method that is relatively cheap and arguably disposable at the end of the winter. Augment this action with all the standard winter preparations of turning off all aeration and turning off or down water circulation – all with the aim of keeping mixing and exposure to the chilling air to a minimum.

Unseasonably warm weather.

The threat of unseasonably warm weather is the sudden cold snap that is likely to close the door firmly behind the warm spell. Your koi may show signs of wanting food – something I’d try and resist – knowing that the risk of a cold snap is very real (and that if your koi have eaten well this summer, then they will have sufficient reserves to overwinter.)


Extreme winter storms of ice and snow may be too much for your bubble wrap option. I suggest that you have a pond heater (100w) to hand to keep a hole in the ice should it form.

Winter koi activity

Water clarity is at its greatest in winter, and as your pond is only likely to have very slow water movement, it will be possible to see your koi very clearly while they are motionless on the pond bottom. Their behaviour and deportment will change markedly over winter. Don’t be tempted to net them and check them over as unless you have an alternative warm / indoor pond – there is very little you can do. It is probably more valuable to check your koi over when spring is definitely upon us as this is when they will be at risk from pathogens, but will also respond to treatments.

Power cuts.

With the energy issues that appear to be topical at the moment, we are told that if we do experience a prolonged winter – then we are likely to experience power cuts.

Fortunately, a power cut in the winter is arguably the best time (if there is one!) for a koi pond to have a power cut. (Unless you are heating your pond). Lots of pond keepers turn off their pond pumps over winter, recognising that as the fish aren’t feeding, there is no need for filtration. There is a disadvantage for doing so in Spring, when the filter has lost its maturity by being turned off. Perhaps a compromise is to keep a reduced flow of water going through your filter to maintain filter maturity. If you experience a power cut under these circumstances then there will be little or no impact on your pond – as long as the filter is not allowed to dry out or run dry for too long. Fortunately, power cuts are usually short. If you are guarding against ice with a 100w heater, then a power cut will mean that your pond will cool down slowly. It will soon recover once power returns via the thermostatically controlled heater.

But overall – don’t panic. I guess the majority of the pond fish in the UK will overwinter without any power being used in their pond.

Jargon Buster:

Necrosis: The death of living cells or tissue eg in fins.

Box Out: What if there’s a power cut?

Don’t panic – the majority of koi and pond fish in the UK will overwinter without any power being used in their pond. If you have opted to keep your pump and filter going at a reduced rate, a power cut will have a minimal impact on its maturity. Check that if during the power cut ice is able to completely cover your pond, clear a hole in the ice with a hot pan of water (from your gas hob!)

Box Out – Top tips for koi health in extremes of winter without a heater.

1. Have a deep pond (4’ minimum)

2. Insulate your pond with bubble wrap

3. Remove aeration

4. Don’t allow your pump of filter to chill your pond water

5. Koi rarely show signs of ill health in the depth of winter – but rather when water temperature rises in spring. This is our period for vigilance.

Winter Pond Information Centre: Overcome ice, frozen ponds and freezing weather safely.

Kill blanketweed and string algae.