In the winter, Koi will usually hang out in the warmer water at the bottom of your pond. Don't add any salt to your pond in the winter either. Salt lowers the freezing point of water. If the water temperature at the bottom of your pond drops below 34°F it could cause injury to the gills.
Here are a couple other things to kind in mind this winter:
It is very important to provide good gas exchange - get oxygen to your fish and allow carbon dioxide (CO2) and other harmful gases to escape.
Leave the waterfall running - Many pond owners will simply keep the waterfall running all winter. The ice formations on the waterfalls are incredible. The action of the waterfall keeps a hole open and provides great gas exchange.
Shut the waterfall down - If you decided to shut down the waterfall, using a floating type heater by itself may not be enough. Because of the cohesive nature of water, we need to disturb the surface. Using a Pond Aerator with airstones is a perfect solution.
Simply plug it in, connect the included tubing and drop the blue airstones into the pond. The force of the air keeps a clear hole and gases can easily escape.
Your fishy friends will thank you in the spring.