Koi are a hardy fish that are more likely to survive through the winter cold. Most koi will be able to survive the winter as long as the water is deep enough that there is room for them to swim around and breathe underneath a layer of ice. Usually, this involves leaving 2 to 3 feet of water. Koi will stop eating once the temperature gets below 50°F. They live off the fat their bodies have put on in the late spring and summer. Their movement will slow down, along with their metabolism.
You'll need to provide a way to keep a hole in the ice so that part of your pond is exposed to the open air. Not only does this allow fresh oxygen to enter the water, but it allows toxic chemicals from waste or decomposition to escape. If levels of CO2 become too high in the water, it can seriously affect your water quality, turning the water acidic and killing your fish.
