Wet/dry will outgas the CO2. There needs to be a cartridge in the slot. After the tank has broken in, I'll probably try that trick of rubber banding quilt batting to the frame. The sump is split into 3 sections. The first holds the filter cartridge, the second holds bio-balls, and the third contains the pump, with a sponge prefilter, and a tube to return the water near the surface.
I like the proportions--the extra front-to-back room is nice. The sump in the back gives me a little overflow protection when I'm working in it too.