Koi are very adaptable and can cope with most climates, although extremes should be avoided. The water shouldn't be allowed to drop to below 4C for long, and rapid changes can be very stressful. This is one of the main reasons why people heat their ponds, not to keep them at summer temperatures, but to keep them stable. Of course people that choose to heat their koi ponds will leave their pumps and filters running, as often they will maintain the temperatures just high enough to continue light feeding. This usually requires that you keep the heat in with a pond cover to cut down on the fuel bills. A cover is also very useful even if you don't heat your pond just to keep out the worst of the winter weather and reduces wind chill.
As the temperatures reduce so does the activity of your koi and filter bacteria. Conversely dissolved oxygen levels increase, so with less demand below 8C aeration from fountains, waterfalls and air pumps can be switched off or diverted. In fact devices like this can actually chill the water further, one of the reasons trickle filters and Bakki showers have failed to take off in the UK. You may wish to keep air pumps running to your filter as moving water takes longer to freeze and it can help with re-establishing the bacteria come spring.
One of the main misconceptions about the design of a koi pond is that it should be very deep for the koi to survive over winter. A lot of this has come from garden pond and water garden advice where fish are kept in a much more natural environment and at lower densities. Most garden ponds are deep enough not to freeze solid, so the main reasons for a koi pond being so deep is its ability to support a greater quantity of fish and also provide more stable conditions. Experts talk about the stratification of water into different temperature layers due to the varying densities that are unique to water. It's because of this that it is suggested that koi move to the bottom of a pond because it is warmer but the truth is that koi sit on the bottom to conserve energy. What is being described is a thermocline where water as it gets colder gets heavier until it reaches it's maximum density at 4C. As it gets colder still it gets strangely less dense until it becomes ice, which we all know floats.
What this means is the bottom of your pond cannot be warmer than 4C before ice starts to form on the surface. On the other hand if you continue to circulate the water from the bottom to the surface the mixing can reduce the overall pond temperatures still further which is not good for your koi. As well as the temperature effect koi need to save energy through the winter months so are better off without having to swim against strong currents.
The answer then is when the water temperatures drop to 8C try to maintain it by reducing the exposure to the colder air by restricting the pump flow or use a smaller pump. You can make the water return to the pond by creating less splashing and it's also a good idea to lag the pipes. Submersible pumps in the pond can be raised if possible to leave the warmer water undisturbed. Years ago before filters were fed by bottom drains it was common to have a mid water intake and are very useful to have if you experience a harsh winter.
If the bottom temperature drops further to 4C then it's probably best to either switch off the pump or use a heater. If you do choose to switch off the pump then try to empty the pipes so there's no chance of them splitting if it does freeze. Don't worry if the pond does ice over as this normally won't cause any problems unless it stays like it for weeks and the pond is heavily stocked.
Labels: pond pumps
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