Ice by itself is not dangerous to koi, as long as your pond is deep enough not to freeze completely (3-5 feet or more).
The danger to fish comes with the reduced oxygen and increased toxic gases caused by the sealed over water surface.
While you could move your fish and plants to a temporary indoor tank for the winter, or install pond heaters that use a lot of energy, in many cases you can overwinter your fish outdoors with a little care. Keep in mind these tips:
1. Aeration: Have an aerator running in your pond at all times to keep the water stirred and promote the exchange of gases. An aerator is a lot more economical than a pond heater, and it will oxygenate the water while reducing ice build up.
2. Ventilation: If you’re aerating the water, all you need is a small hole or two for the bubbles to escape. You don’t need to remove the rest of the ice from the surface.
3. Balance: Koi (and many other fish) go into a sort of dormancy in the winter, using less oxygen and naturally settling to the bottom of the pond where the water is warmer. Your aerator should gently stir the water without upsetting this natural balance. Go for a steady stream of bubbles, not a rolling “boil” or strong current. If your pond is 4-5 feet or deeper, put the diffuser at the middle of the depth range (not the deepest part) so that the fish can retreat to less-disturbed deeper waters if needed.
4. Ice: Don’t break the ice in your pond by hitting it, since the force can cause shock waves that may hurt the fish. Keep an eye on your ventilation hole and remove any thin ice before it becomes thick and hard. If necessary, you can sit a full, hot teakettle on top of the ice until it melts a hole. Otherwise, let it snow! The snow, ice, and surrounding earth will help insulate your pond from the frigid air.
5. De-Icer: If your pond threatens to freeze completely, you may need to install a de-icer to keep at least part of the water above freezing. Your fish will need a few feet of liquid water in the pond throughout the winter.