Not sure..If the pond is deep enough, you don't need to worry about it, but what we do is take our koi into the house during the winter and put them in this huge beautiful pond we have in the basement.
If you live in the mid-west where you can get down to -20 below in the winter, you would need a 2 1/2' to 3 foot pond to be safe. My ponds are just over 2' (28 inches), and I run a "pond de-icer" with a bubbler attached about half way down. No problems at all.