Actually, we need some more information here.
For one, unless you have an astronomical sized pond, you can't support "100's of free Koi." Koi get very large and need excellent, top notch filtration and water quality.
Secondly, just what do you have to keep the fish in? Do you have a pond? What kind? How many gallons and how deep? What is your climate where you live? If your pond is at least 3 feet deep and wouldn't freeze solid in your climate you really don't have to heat it. You only need to keep a hole open in the surface ice to allow for gas exchange.
Depending on what you have for a pond, the stock tank heater may work but you will notice it on your electric bill. They are electicity hogs.