Heating a pond in winter

What’s best for your koi in winter? Do I heat my pond or not?

We mustn’t forget that koi are a warm water fish and are tolerant of a wide range of temperatures – making them an ideal pond fish. Don’t presume that koi cannot over winter at ambient temperatures as the majority of koi in the UK do so satisfactorily each year without any human intervention. In fact, a koi’s physiology is geared to overwintering. The shortening daylength as winter approaches causes koi to prepare for winter before it arrives, feeding in excess (whenever possible) to store sufficient energy to survive the winter. The onset of cold weather may come as a shock to us, but our koi, unbeknown to us, will have been preparing for this testing period for months.

Winter Pond Advice.

All you need to know for your pond and pond fish to survive the harshest winter in a garden pond.

Dear Ben, I know it is not quite autumn yet, but as a very new koi keeper I am trying to be extra careful and think well ahead of myself.

My husband completed our koi pond in March this year, and we have enjoyed our three dearly loved koi throughout the summer months. However, I have heard conflicting opinions on how to adapt to the winter months, and am concerned that I might do something wrong that will endanger our koi.

If you could give me some advice now on heating the pond, filtration, and any other pieces of equipment that may need altering, I will be able to prepare myself for when winter hits us. Thank you very much for a great magazine.

Your overwintering strategy will depend on specific aspects of your pond (such as depth) as well as what you are looking to achieve with your koi in a freezing pond over winter. I’d love to be having this conversation with you standing next to your pond so we can see exactly what you’ll need to do (if anything) to take your koi safely through to next spring. However, this isn’t possible, so where necessary, I’ll have to make some assumptions that I hope will be accurate.

What’s best for your koi?

We mustn’t forget that koi are a warm water fish and are tolerant of a wide range of temperatures – making them an ideal pond fish. Don’t presume that koi cannot over winter at ambient temperatures as the majority of koi in the UK do so satisfactorily each year without any human intervention. In fact, a koi’s physiology is geared to overwintering. The shortening daylength as winter approaches causes koi to prepare for winter before it arrives, feeding in excess (whenever possible) to store sufficient energy to survive the winter. The onset of cold weather may come as a shock to us, but our koi, unbeknown to us, will have been preparing for this testing period for months.

As soon as the water temperature drops below 8oC, koi become relatively inactive, drop to the bottom of the pond, not to feed again until the temperature rises above 8oC in the spring. It is the length of period for which koi are inactive that we do not have control over and we have to accept the winter that materialises unless you choose to intervene.

Aren’t inactive koi at risk from disease over winter?

You are unlikely to witness diseased or unhealthy fish during cold and icebound conditions but any problems that do occur are more likely to manifest themselves in the spring when water temperatures and disease populations can explode ahead of the koi’s ability to fight them.

What about heating?

Your interest in heating your pond over winter is one strategy that will give you control over the length and depth of winter. It will also help you control your pond temperature as spring approaches, helping you to bring your koi out of their period of inactivity predictably, returning them eventually to normal summer temperatures. If you are looking at heating you pond, then I assume it is 4′ deep or less because any deeper and your koi should be able to over winter satisfactorily.

But before going ahead with heating, 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.

The benefits of heating

Total control over the water temperatures that your koi are exposed to, even in the harshest of winters.

Unpredictable and wild swings in temperature at the onset and end of winter can be ironed out, again protecting these artificially selected carp from the trials of nature. Unlike the natural world, we do not want the winter period to subject our fish to a test of a survival of the fittest.

If koi activity is maintained to some degree, then they will be able to resist disease better by having an active immune system.

Your filter will not experience a period of inactivity, removing the need to jump-start it in the spring – something that could run the risk of a water quality problem.

The Costs.

Depending on how do you choose to manage your water temperature over winter, you run the risk of confusing certain koi bio-rhythms, which may go on to affect their spawning activity in the subsequent year.

Your koi pond will not benefit from a fallow period. Instead, koi parasites and other pathogens will remain active throughout the year, putting your koi at a greater risk of disease.

Cost. There is both a capital outlay for the heating hardware (and its installation if required) and also a significant running cost. You will have to compare this against the potential benefits before making your decision.

Electric Heating – Your simplest option. (I am assuming your pond is no greater than 3000 gallons)

To avoid the release of any potentially harmful metals into your pond, be sure to choose a pond heater that is manufactured specifically for heating koi ponds. The most popular electrical heaters are plumbed in line between your pond pump and return pipework (as per a UVc) and just like a UVc, every care must be taken to ensure they run at full bore. The wattage of the heater is determined by the volume of the pond that needs heating. The general rule of thumb is that every 1000 gallons of water requires one kilowatt of heating power. Up to three kilowatts, heaters can be plugged into the standard mains supply via an RCD or RCBO (circuit breaking device). Heaters above 3Kw will need to be wired back, directly into your house consumer unit (or fuse box).

What about filtration over winter? In winter with no food entering the pond, ammonia and nitrites are not likely to reach toxic levels. DO is not an issue either as the metabolism of those organisms consuming oxygen will de depressed. However, pH can be a cause for concern and should be monitored, particularly in the early spring during the thaw. But if you do decide to heat your pond, you will have to continue filtering as before.

You can choose to use your heater to provide either summer temperatures through the winter months or to simply remove the harshness of the winter by providing your koi with a steady 10C (or so) all 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. An alternative, cheaper strategy is to keep your pond at 10C. 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.

Alternatives to heating a pond?

Those koi keepers who do heat their ponds over winter do gain the benefit of seeing their fish feeding and gaining 365 days enjoyment from their fish. However, there is a cheaper alternative that does not include the expense of heating and success can be achieved economically by insulating a pond. In this way the koi experience the helpful influences of a winter, but are not exposed to the risks associated with a full-blown, fully exposed winter.

Bubble-wrap option

This is one of the most practical, economical and reliable ways of preventing your koi from being exposed to the risks of a full-blown winter. A simple frame or hooped arch can be constructed from over-flow pipe and push-fit connections onto which a layer of bubble-wrap can be attached. Heavy-duty bubble-wrap is very affordable and can be used for several years depending on how well it is attached to the hoop structure. Its life will be shortened if wind is allowed to get inside the ‘tunnel’ and blow the sides out. This form of insulation can be used in conjunction with a small 100w pool heater, installed through a greenhouse-type thermostat which can be set to come on during frosty nights.

Although it’s not the most aesthetic means of overwintering koi, it can easily be dismantled and re-used each year and has a proven track record of protecting koi and ponds from the harshest of winters.

Stability, predictability and the avoidance of extremes

The key issue about over-wintering koi safely is stability and the avoidance of extremes. The stability of water temperature and water quality is essential, ensuring that any changes that do occur are gradual and within the natural limits of the koi.

This is quite easily achieved in bigger ponds that are deeper and larger in volume but can be a problem to all koi keepers whatever their set-up either side of winter when they are subjected to isolated warm spells in September, October, February and March.

By installing a heater and adopting a winter heating strategy that takes into account the overall well-being of your koi, you should be able to overwinter your fish with confidence, without there being any implications for their health, growth and spawning the following year.



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