I have a cold issue in my house and the only solution I have come up with was to get a proper cage to retain heat I have a uth set up with a thermostat and I use an oil heater that's set up to a thermostat as well. You can try insulating the outside of the tank with Styrofoam. I had a friend do that in the past because he had a class tank that didn't hold heat well.

Hope you can figure it out.