An effective winter pond strategy for your koi, goldfish and plants.

Q: This will be my first winter since building my koi pond in April. My pond is relatively small, about 1,000 gallons and it has an Aquamax 5,500 pump with a 20 watt UV and small pump-fed filter.

Since it was built, the pond has reached temperatures of around 70F in the summer with no heating but would it be advisable to keep it this temperature during the winter? Would I need to keep it heated and covered? How much would this cost me?

Or would it be better to just let the pond freeze over so the koi take shelter under the ice? I’m just not sure what would be better for the koi?

Any advice/suggestions would be gratefully received

A: As I write this reply to your letter, we are a few days short of October and there is still no sign of a winter. This time of year is typically wheatgerm territory, where the water should be cooling down to about 12C, but this year, ponds are still unseasonably warm and koi are still eagerly digesting a higher protein growth diet. This is good news for your koi, as it means they are able to store more energy for their fallow winter period (if you decide to let them have one) and also means that they are likely to spend a shorter winter period without feeding. Surprisingly, despite the unseasonably warm weather, your koi are not oblivious to the oncoming and imminent winter period as their sensory system would have been tracking the shortening daylength, preparing their bodies for winter by storing energy instead of utilising it for growth as they would have done earlier in the season at similar temperatures.

Your letter asks a similar question and that is ‘how should you prepare your pond ready for the winter so that your koi come through to next spring unscathed and in good health?’.

Heat or not to heat?

The number of koi keepers (and pond keepers) heating their pond is definitely on the increase. This is largely due to two factors: A change in opinion on how koi should be overwintered and the increased availability of inexpensive, easy-to-install heating equipment.

The changing opinion on overwintering. Koi and most other pond fish are essentially warm water fish whose ability to tolerate a wide range of temperatures is exploited by pond keepers from January through to July. This strategy, of letting our koi and pond fish experience and the UK’s seasonal calendar was (and still is) supported by well-defined periods in the year where our koi were offered a high protein growth diet in the summer and lower protein wheat germ diets in the spring and autumn. Such a feeding regime enables koi to grow and store energy in the summer and gently introduces them into their period of inactivity in the colder months. Once our koi had stopped feeding we had to leave koi to ‘go it alone’ and hope that Mother Nature’s relationship with Jack Frost was a short one. We tried to increase their chances of success through the winter further by providing them with ponds of at least 3 ft in depth, preventing ice from totally covering the pond by using an inexpensive aquarium heater look-alike called a pond heater.

I know of pond keepers who continue to adopt this overwintering strategy in their ponds after many years of successful koi keeping. I have even heard of a koi keeper who has kept his koi in a raised, temporary above-ground converted paddling pool for years without experiencing winter mortalities. Have we all just been lucky for decades? Have our koi really suffered by being left at the mercy of the winter weather? I suggest that the vast majority of koi and pond keepers will continue to adopt the traditional approach to pond keeping overwinter by not intervening and on the whole continuing to be successful.

However, with the onset of pond heating, I also suggest that in certain cases, I would recommend that heating should be considered. But beware – even though I suggest that it should be considered, there are certain factors that you should take into consideration before and during heating your pond. Heating certainly does provide you and your koi with tangible benefits, but you should be cautious if choosing to use it in certain ways.

In your letter you describe that your pond is 1000 gallons in volume. Unless your pond area is less than 50 square feet (which is unlikely as it would make a very small pond) then by simple arithmetic, your pond’s depth will not be deeper on average than 3 feet. This is on the borderline of providing your fish with sufficient insulating protection against the ice-cold winter air. For two reasons, I suggest that you consider installing a heater for this winter. Firstly, because of the limited depth of your pond, and secondly, because a heater is just as easy as a UV to install, and is something that you have already installed in your pond.

A 1Kw pond heater for 1000 gallon pond is all that you would require, and would easily run off a three-pin mains plug (as opposed to having its direct feed from your consumer unit as say an electric shower or a cooker would). The heater itself is installed in a similar fashion to the UV, and will only cost you approximately 300, including cable. Once you have your heating installed, you then have three options on how to run it over winter.

Option 1 – Maintain Summer Temperatures Throughout Winter

As you suggest, one approach is to maintain summer-like water temperatures throughout winter.

However, koi physiology responds not only to temperature but also to day length. During a temperate winter, shortening daylight hours trigger a natural reduction in growth hormone secretion. As a result, even if koi continue feeding through winter, they will not grow significantly in length. They may gain weight, but true structural growth is limited.

Maintaining summer temperatures year-round therefore does not replicate natural summer growth rates. Instead, it results in high running costs and increased feed expenditure, without delivering proportional benefits.

In my view, such a strategy is not only unnecessary but may also lead to spawning complications in the following year.

Both day length and water temperature influence the ripening and development of eggs within female koi. As spring approaches, increasing daylight combined with gradually warming water signals the onset of summer, triggering spawning behaviour.

If koi are denied the natural seasonal peaks and troughs — similar to those experienced in regions such as Niigata, Japan — they may fail to recognise the seasonal transition into summer. This can disrupt normal reproductive cycles, potentially leading to koi becoming spawn-bound later in the year.

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Option 2 – Maintain a Steady 10°C Through Winter

A more balanced strategy is to reduce winter harshness while still allowing a seasonal slowdown. Maintaining a steady temperature of around 10°C provides protection from extreme cold while preserving a sense of winter.

This approach offers significantly lower running costs than maintaining summer temperatures year-round. At 10°C, koi can be fed a light maintenance ration of low-protein food, allowing both fish and biofilter to remain moderately active.

From discussions with koi keepers across the country, this appears to be the most widely adopted winter heating strategy.

By maintaining water at approximately 10°C:

• The immune system remains partially active.

• The biofilter continues functioning.

• Disease pressure from bacteria and parasites declines naturally in cooler water.

• Fish are protected from dangerous temperature extremes.

Importantly, this strategy retains a seasonal temperature differential, even if artificially controlled. Koi still experience winter, followed by a gradual spring warm-up — helping preserve natural biological rhythms and supporting normal spawning behaviour.

In essence, this option strikes a practical balance between fish welfare, cost efficiency and biological realism.

Option 3. The final option is my preferred option. This takes option 2 a stage further by providing your koi with a controlled cold period (4-6C for two-to-three weeks) sandwiched either side by a controlled period of 10C (as per option 2). This will lead to even further savings on your heating bill as well as a reduction in your food costs as your koi will stop feeding altogether below 8C. This strategy is still risk free for your koi, as you retain control over your water’s temperature even during the coldest period, by setting your heater’s thermostat to say 5C. Furthermore, and most importantly your koi will experience and benefit from a winter period with proven and documented benefits for their subsequent growth and spawning success. A cold period resets your koi’s biological clock and increases the chances of spawning. Many Japanese koi farmers also report that growth in koi in spring is far greater in fish that have experienced a winter compared to those that have not.

Extra cover.
Which ever heating option you choose, your heating costs can be reduced by adding strategically positioned insulation on your pond. Your largest area of heat loss will be from your pond’s surface, so a simple timber frame with a pitched roof, covered with bubble-wrap as close as possible to your pond’s surface will save you pounds over the months do you choose to heat. The pitched roof will not allow rain to settle on the bubble wrap, thereby keeping it clear of the water. Problems of gas exchange between the pond water and atmosphere may result if you allowed bubble-wrap to lie flat on the pond’s surface. You should also insulate your pump-fed filter, together with the pipe work leading to and from the filter. I would also encourage you (if your filter feeds a waterfall), to temporarily extend the return pipework directly into the pond to avoid your return water being chilled when it comes into contact with the freezing air.

In summary, from your description your pond’s sounds as though it is on the borderline of being able to guarantee the safety of your koi overwinter. By installing a heater and adopting a winter heating strategy that takes into account the overall well-being of your koi, (and your bank balance), you should be able to overwinter your fish with confidence, without there being any implications for their health, growth and spawning the following year.