A closed loop by definition means you are pulling water out of the main tank and pumping it right back in. You don't want your closed loop moving water though anything other then the glass, piping and the pump. A closed loop should have it's own seperate intake and typically it is drilled in the back glass or bottom of the tank. The returns are placed/drilled where ever you want and in what ever configuration you want. Bottom line 1 hole in the tank for the pump to be fed and at least 1 hole in the tank for the return line.
I use 2 1200gph hang on the back overflows (Eshoppes). My tank is not drilled and I did not have time to deal with drilling it when setting up this tank. Read through my thread you will understand. Having a Reef Ready tank is the way to go. It is predrilled and has the overflows built inside the tank. Plumb them up and you are ready to go.
The drawing above showing a return from the sump towards the middle of the tank, that is a disaster waiting to happen. If your return pump fails it will drain your tank to that level and flood your house. Remember the return/sump/overflow is not a closed system and can cause a flood if plumbed that way. Returns are typically towards the top of the tank and have some way of breaking siphon if the pump is shut off. Mine are plumbed with PVC
and I drilled holes in the PVC
right above the water level of my tank. When my pump is off the siphon breaks immediately and the water level barely drops in my tank.
I hope that helped...
I was under the impression that a fuge should have 600gph or less moving through it. The point of the fuge is for maximum contact time with macro, LR