I have a fishhouse in my back garden. the building is 12'x12' and is insulated with rockwool and polystyrene. the inside measures 10x10 because of the insulation.
I have a heater in each tank and there is a 2KW oil filled radiator. the air temp is kept constant @27C and the tanks stay around 26/25C some of the tanks near tyhe floor need an occasional "boost" by the aquarium heater in the tank itself to keep it up to temperature.