+1 Fishward you need about 8 gallon min for return. i have a small return in my temp sump while I finish my 40g the ATO goes off about every 1.5 to 2 hrs. To does not bother me as the tank does not over flow and I have not seen any determent to it . but larger is setter. you an make it larger by taking out a couple baffles all that is needed is and over and under you can still have a second over and get a larger return area out f the deal.

Bill