Yeah, i was going to say 400 gph is way too low for that kind of sump with 2 overflows. I'm running a 600 gph overflow and a 700 gph pump but by the time the back pressure on the pump is taken into consideration its about right. You could have a less powerful pump than your overflow, in fact I think I would rather have a smaller return pump than my overflow. so basically all the water that gets put back into the tank will overflow instantly, so if your returning 1000 gph, only 1000 gph will be able to overflow into the sump. (not sure if you get it, im attempting to make sense here, but i'm not an expert) Hopefully someone will verify my hypothesis here, with the overflows it cant flow more than the pump can handle returning if the water isnt high enough to overflow... And if you have a return pump that is doing too much work, you can throttle it back with a ball valve, or like Biff suggests, make a T with a ball valve to let the excess water back into the sump, instead of just making more pressure.