Winter, the coldest season of the year. If you live in the nether regions, then you probably get snow and ice. Koi go dormant in winter, so they do not eat or produce any toxins. Do not feed your Koi if the water temperature is below 50°F. Food will sit in your Koi's stomach and rot.
It is a good idea to keep a space open in your pond for gas exchange. Carbon Dioxide needs to get out of the water and fresh oxygen needs to get in the water. This can be done with an aquarium powerhead and/or an airstone. A horse trough heater can be used too, but they are expensive to run. It is also a good idea to put an airstone in your pond to supply your Koi with fresh Oxygen. Put the airstone near the surface so that you do not mix the pond water up.
You will probably notice your Koi sitting on the bottom of your pond. This is normal, Koi are Poikilothermic (cold-blooded), so their activity level coincides with the water temperature. In the winter, Koi will go to the warmest area. Since water is at it's densest at 39.2°F, the warmer water will be at the bottom of your pond.
Make sure your water temperature doesn't drop below 34°F. If it does, ice crystals will start to form on your Koi's gills. Ice crystals can kill a Koi. Don't add any salt to your pond in the winter either. Salt lowers the freezing point of water, so if you add some to your pond, it could possible kill your Koi because the water temperature could drop below even 30°F.
It is also a good idea to shut off any waterfall. The only thing a waterfall will do in the winter is lower the water temperature. A waterfall could even freeze up, causing all of the water to pump out of your pond.
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