Loretto, the bio balls are a nitrate factory for a reef. The BioCube is for fresh and saltwater. The back chambers are set up as a wet/dry filter. If you were doing a fish only no live rock then use the bio balss. Without the bio balls you can fill chamber 3 all the way to the top and not have to worry about the fill line. HTH.
I run it with live rock in chamber two with a heater. All my water parameters are stable.
I am too interested in the fuge set up. I have a small ball of cheato behind the live rock in the tank and would love to put it in the back. However, I don't want to do a lot of mods.
Tom