Convict2161
Aquarium Advice Addict
Wouldn't the rubble trap a bunch of detritus too? Do you need to clean it? Why put LR in there when you have a bunch in the display?
Live rock will grow BB just like those plastic balls. Your just better off with the live rock. You don't have to clean it. Some even put a crab and a few snails in there.
Also you can make it a fuge. Even with the live rock you can add the fuge light from the outside and make it like another small tank.
I've seen people use the back with live rock, a small CUC, light and even some corals. It's up to you. Removing the bio balls also helps with flow.