Winter pond routines part 1 – Heating

Winter can be a very boring time of year for koi keepers, especially if you don’t have an additional indoor pond for growing some koi on over the colder months. Although it can be very long and drawn out it is in my opinion an absolutely crucial season for koi, and one that will play a massive part in their overall development.

Everybody seems to have different opinions as to what are the best conditions for koi in winter so I thought I would look at how I prefer to keep koi at this time of the year and explain my thinking behind the methods I choose. It’s a known fact that koi have adapted to cope with winter and research has shown that it can be stressful for koi if they don’t spend time in cold water (around 7°C and below) and with periods of less than 10 hours of daylight.

Winter should be a resting time for koi and a period where they are able to recover from the summer growing period

So onto the topic of heating and I suppose the first question is ‘to heat or not to heat’?

Well, I am an advocate of heating, but only because my optimum water temperature for winter is 7°C and even when an outdoor pond is covered it’s very rare that the water will be able to reach this temperature. Now I am not saying that the water must be 7°C because koi will live perfectly well down to about 3-4°C, but ideally you want to avoid the koi spending prolonged periods of time sitting on the bottom of the pond which they will naturally do if it becomes too cold. It’s also a good idea to have a heater installed, even if it is not used, because in the event that you encounter any problems with your koi during winter you will be limited to what you can do. Having a heater will enable you to lift the temperature while you sort out the issue and then drop it back down afterwards.

If your preference is to keep the pond warmer than my optimum temperature then the absolute maximum I would recommend would be 12°C, I think it’s important to remember that winter should not be a time to try and grow koi unless they are tosai. Winter should be a resting time for koi and a period where they are able to recover from the summer growing period, get their skin back in top condition and burn off some of their fat reserves to prepare the body for the growing season the following year.

My last comments on this subject are regarding covers because it is now common to cover outdoor ponds during winter, whether heating is being used or not, and it is a very good idea to reduce heating costs or simply to prevent the temperatures dropping too low. However, I must stress the importance of firstly making sure your covers are made from a material that will allow daylight through, and secondly making sure you have a period each day with part of the covers lifted to allow gas exchange to take place. Having the entire surface of the pond covered will not only stop gasses such as carbon dioxide, ammonia and nitrogen from getting out, but will prevent oxygen from getting in, even if you do have an air pump running. Failure to provide daylight and good gas exchange could lead to health problems and bacterial blooms (especially for those with higher temperatures) so keep this in mind when you design a cover for your pond.

In my next post on this topic I will be exploring different feeding regimes for winter and, in particular, whether or not it is a good idea to give koi a fasting period.


  1. Good stuff Ricky i am dropping mine down a degree or 2 each week, now at 13 will take it to 8 degree. How long do you give them without food or do you feed all year round.

  2. Interested in water changes recommended at low temperatures, not sure if my message to you got through.

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