I'm trying to design an overflow for a 120 gallon acrylic tank. It will be eurobraced w/ center overflow. However, I'm trying to figure out the overflow. I'm thinking two 1 inch drains with two 3/4 inch returns. I'm not planning to push much flow through it, 600-800 gph. How are internal returns designed? Can I have holes on each side panel of the middle overflow drilled with a hole about 20 inches high for a locline return? In case of a blackout, would check valves prevent a flood from the returns? Any insight would be greatly appreciated. Thanks.