A water trough heater from the feed store is just as good and cheaper then some of those pond de-icers. Ponds need to be no less then 3 1/2 deep or better for koi to winter over well, even then, some do not. If you see red fins, laying over, then they need to be brought inside. The alternative is to buy a stock tank from a feed store and set it up in a garage, out of the weather. One of those would give them more room, stay a bit warmer and be easier to set up and care for. The pond pumps can be used with a stock tank!