As the winter months approach the water temperature in your pond begins to fall, as does the fish’s metabolism. From now on the fish will be entering a state of semi-hibernation. It is very important that during this period, undigested food does not linger in the body. Once the water temperature drops below 65 degrees do not continue to feed your fish high protein food.. You should now consider feeding them good quality wheat germ based food until winter is upon you in earnest. At this point, no further food, what so ever, should be offered to the Koi until the following season. See here for temperature feeding guide. Don't worry, your fish will not come to any harm, they will live on natural food from with in the pond.

One other area of concern in winter is the formation of Ice on your pond, which will trap gases and other toxins underneath to the detriment of your fish. Folks who have left their traditional backyard ponds covered with ice for weeks have lost entire collections of fish. It's hard to believe that there could be that much gas formation in the dead of winter, but the proof is in the experiences of hundreds of people every winter. So, keep a place in the ice clear for gas exchange and observation of the fish. Cattle water trough heaters (caged heaters) or specialty pond heaters are cheap (about 30-50$) and can keep a patch of ice clear all winter. Do not break the ice with a concussive blow, in the event that you are caught unprepared and you find your pond frozen. The blow to the ice is supposedly transmitted through the water and will shock and possibly injure your fish. Take your time, you have days, even a week to open a hole in the ice. Use a hot teakettle, set directly on the ice.

As winter approaches it is essential that your pond system is in the best condition possible. You should take this opportunity to carry out basic cleaning and tidying up procedures in and around your pond area. By doing this you not only ensure your fish are in the best possible environment for the coming months, but it will also give things a useful kick start for the following spring. It is very important you carry out these basic cleaning operations before the environment become dormant.

If you choose to switch off pumps and shut down filtration systems for the winter, you should fully drain, clean and store in a dry place. Pumps that are left in the pond should be lowered to such a depth so as not to be frozen solid in any ice that may occur. The pump should be run from time to time to keep the bearings free.

If you choose to run your system throughout the winter, (Depends on the weather in your area), you should raise the pump nearer to the surface and closer to the water inlet of the pond. Reduce the turnover of the pump so that the water is just passing through the filter slowly. This will keep things ticking over nicely. Do not allow the pump to be encased and frozen in ice.

A hardy winter is not all bad news, it can have a beneficial side to it. Parasites that the fish may be carrying will be reduced, in turn preventing infestation the following season.

During these colder months, fish will tend to seek shelter at the bottom of the pond where the water is warmer. If at some stages on bright days through the winter months you see the Koi hovering close to the surface, do not be tempted to feed them, as their digestive system is still operating at a reduced rate.

Winter Preparation For Your Plants

Once plants start to die back, you need to remove dead foliage. Take care with any plants that have hollow stems. If you cut them back to far and allow water to enter they have a tendency to rot away. Any plants that are required to be over wintered in warmer condition should be removed from the pond before the first frost, or the chances are they will die.

Water lilies should be cut back as close to the base as possible, do not let leave old foliage to decay and rot, as this will cause toxic gases to be released, leaving your fish gasping for breath. They can now be lowered to deeper water where they will be safe from any frost.

Do not forget to raise them to a suitable flowering depth in early April the following year.

This is also a good time to replenish the old soil with new soil. If you can lay your hands on soil from mole hills, all the better, as moles are only interested in finding worms, so will only dig in soil rich in humus as this attracts worms. This is why soil from mole hills make a good medium for growing waterlilies.

A good idea is to cover your pond with a net, this will not only prevent falling leaves and other winter debris from entering the pond, eventually rotting and releasing toxic gases. It also prevents HERONS and other predators from preying on your much loved Koi.