i wouldn't make a second shelf there personally since other media is supposed to be submerged. the water level of the setup will only drop in the sump not the tank so you'd have to stay on top of the water level more so if it's raised. i have that entire area under the bio balls completely packed with media and i also raised my bio balls a few inches to allow more media and water (i work with acrylic for a printing company and made risers for the egg crate holding the bio balls and the drip tray). i'll take a couple pics of my sump setup later when i get home









