Dumb question but I will soon be setting up a 120 gallon tank with the kind of filter made out of one of those 3 drawer sterilite units sitting in a sump (if that is the right term). I figure I will need 2 heaters for the volume of water..will it be effective having both of them in the sump or does one need to be in the tank? Sorry I've never built one of these before and I'm a not confused. Thanks!