Its simply an overflow. The positive side to this design is if there is a faliure with the overflow, there wont be much water on the floor and any repairs are easy to get to. Also no holes in the stand, no worries about cracks in the bottom of the tank around the bulk head fittings. The negative side is the overflow pipes inside the tank are kinda ugly and cant be hidden. Also the plumbing is very visible from the back of the tank, as you must leave about 12" from the back of the tank to the wall.Gr8 update
One quick questionhow does your inlet draw water down to your sump ?
Never have a problem with nitrates, even when I get lazy and skip a WC (not recommended)How's that pothos vine helping with nitrates? Very cool set up and stock btw.