The recent cold period over here pushed our quarterly bill from its usual £170 to £620 and most of that cost is heating the fishroom, its a vicious circle, the room needs ventilation otherwise it gets damp but ventilation means heat escapes (and cold gets in) so the heater is on for longer to keep the room at 78f, if i had halide lighting and several powerheads to add to that i hate to think how high it would go!!