I used to run a purpose built fishhouse with about 50 tanks which were grossly overloaded with fish and required massive and frequent water changes. It was heated by a fan that ran continuously and a 1kW heater that was on about 25% of the time. 100mm of poly in the floor and batts and sisalation in ceiling and walls. At the moment we have a number of tanks in 3 different places around the house and garage (4 x 4 footers and a few breeding tanks) and the power is about $150 - $180 summer and $200 - $250 winter. Yhe house is uninsulated other than batts in the ceiling. Heavy insulation cuts the power consumption to hell. If I was doing it again I would build a room for that only, insulate heavily and use the heat from water going out to heat the water going in. Heaps of tanks helps as well as it acts as a heat sink. There is not much energy in the air that leaves when you open the door but there is a lot in the water that stays behind, or goes to waste with water changes.