Winterizing a Backyard Fish Pond

This will be my first winter since building my koi pond in April 2003. 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

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 a koi pond. 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. As you suggest in your letter, provide your pond with summer temperatures all through the winter.

A strange thing happens to a koi’s physiology during a temperate winter wherein when the day length shortens in winter, so does a koi’s ability to grow. This is caused by a reduction in the secretion of a growth hormone, meaning that in effect, even if your koi feed over winter they will not grow – (they may put on weight, but they will not put on a significant increase in length). So by providing your koi with a summer climate all year round, you will not experience similar summer growth rates (and yet you will be experiencing very high running costs, and higher food costs throughout the colder months. In my opinion, not only is such a strategy unnecessary, it may lead to spawning issues and complications in the following year. This is because both day length and water temperature are key factors in the ripening and development of eggs within a female koi’s ovaries. Koi detect a warming of the pond water as summer approaches and that, combined with a lengthening day length, will lead to a spawn. If you deny your koi and the peaks and troughs of a normal year (just as they would experience say in Niigata, Japan) then there is an increased probability that your koi will not in effect detect summer and not spawn at all, leading to koi becoming spawn bound in the following autumn and winter.

Option 2. Remove the harshness of the winter by providing your koi with a steady 10C all winter.

This strategy provides you straightaway with cheaper running costs than the previous option, while also providing your koi with a risk-free winter. At this temperature, your koi could still be offered a maintenance ration of a low protein diet, enabling them to feed and your filter to still function over winter. From speaking to koi keepers across the country on this topic, this appears to be the most common strategy adopted by those who heat their ponds over winter. By reducing the temperature for that lengthy winter period, you also reduce the threat of disease from bacteria and parasites (as their numbers will decline naturally) while your koi’s immune system will still enjoy a degree of activity. It also provides your koi and pond with some sense of season, even though the temperature differential is produced artificially.

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.

Kill blanketweed and string algae.