The middle of the box, when its running, bows in and back out after the flange passes. It's actually nice having the space between so it's easier to siphon, but i have a hard time cleaning the glass without fragging everything. Luckily, i've only had to clean the glass like 5 times since the tank has been running(nov).
I wanted to have my rocks lifted off the glass, but got lazy. That blocks a bit of flow from going underneath, and I guess keeps the lps pretty happy. I have some cloves(clavularia sp.) mounted on the top of the box and a frag of lithophyllon, and both are very happily growing.
joe