I youst to have 1 250 watt heater in my 120g 30g sump with it the overflow and it never turned off way to mutch water to heat up and i keep mine at 77 to 79 i added another run 2 250 watt heaters and works great but mine is 120g and 30g sump, so maybe 2 or if not 3 but thay make 500w and 1000w heaters most have a external controler but that all depends on lighting return pumps how many pumps you run and anything in tank that heats water, most say youse 250watts per 75 gallons