what kind of tank do you have? fresh i'd assume. but it always helps to check. And how many fish, and what species? Part of the function of a sump is to balance ph at night, which is why you have plants and and a light that turns on at night. Of course it also increases water volume for your tank, and if you want to overstock your display you sump will help keep everything in check. Honestly, a 5g wouldn't hurt, but what it helps would be minimal. I'd do at least a 10g. You can put your heater in the sump, that would keep it out of the display tank, but you don't need the sand, cheato, or lights if you don't have a salt tank.