Bioballs can be just as effective, if not more so than LR
rubble if they are properly cleaned and maintained. Doing so requires that clean a 3rd of the bioballs at a time on a regular basis by cleaning/rinsing them off in freshly mixed saltwater. The notion that LR
rubble provides more surface area for bacteria than the bioballs is a myth. As is the notion that LR
rubble will not trap debris and become a source of nitrate over time. Studies have shown that both LR
and Live sand can eventually begin to absorb nutrients when they are present in high concentration in an aquarium and begin to slowly release them back into the system. Anyone who has ever had LR
rubble and cleaned it out at some point will tell you for a fact that all kinds of junk gets trapped in or under the LR
rubble. IMPO, regardless of what you place in the sump, it needs to be removed and the sump cleaned out on a regular basis.
The one real difference between bioballs and LR
rubble is that LR
rubble provides anaerobic spaces for denitrifying bacteria to live and grow, so your LR
rubble will actually help to convert the nitrate produced by nitrifying bacteria into atmospheric nitrogen. Provided that you don't have cyanobacteria populations in your sump/tank that can trap and convert that atmospheric nitrogen into a usuable form, it will gas out.