I had a purpose built fish house in a previous life. The most imprtant thing is to insulate the hell out of it. I then got a 2bar fan heater and wired it up so the fan went all the time and 1 bar (1kW) heater was wired through the thermostat. The heater hardly ever came on. The next thing is to get heaps of tanks and the water acts as a heat sink. When you open the door the hot air escapes but is easy to re heat. The heat is maintained in the water, so the more you have the better.