A paintbucket heater can be wired onto a Ranco HVAC thermostat. I did it, you could too.

Keywords: Koi, Goldfish & Pond Health Heating Koi Ponds and Water Gardens Electric Propane Natural Gas Koi Heaters Koi Heat

Warming a fish up one degree per hour - and no faster - how to start the process, especially in winter:

Fish in winter ice water sometimes need to be warmed up for various treatments or to rescue them from genuine cold water illnesses such as Laying Over. To avoid shock, the fish should be put in a very large vat of this icy pond water in your garage. I repeat, you're going to use the icy pond water from your pond, that the fish is used to. This vat should be at least 75 gallons in size or the temperature will equalize too rapidly and kill the fish. They cannot climb up more than 10 degrees F per 18 hours without serious stress or death. Let the vat slowly warm to garage temperature using the ambient room air. USE NO HEATER. Do nothing to accelerate the warming process. It's meant to be slow! As the fish warm up, they will become more active. Make sure the vat is covered. Once they have been in the mid sixties for 24 hours, you can raise the temperature with a commercial aquarium heater or a paint-bucket warmer, by five degrees per day til in the low seventies.