I use 3 in my 125 (really 150g total volume). 200 and 300 in return chanmber, 200 in tank. The in tank one is really just in case the pump were to stop for some reason. My basement is unheated, so it's in the 50s much of the time in the winter. Even a tank my size would cool down to fish killing temps in a couple of hours with no heater, I think.