While some would suggest LR "rubble" as a replacement for the balls in the W/D, I would suggest nothing at all...
Reason being the balls purpose is to provide surface area for the bacteria to grow and provide biological filtration...
That "purpose" in fulfilled by the LR (and sand) in the tank...
The problem with the balls is they can (being in an out of the way location) become a "trap" for deitrus, which then decomposes and "adds" to the NitrAte buildup in the tank...
My thought is that the LR rubble replacement will have the same effect if not cleaned often (as with balls)...
So by eliminating this deitrus "trap" from the system, you prevent the decomposition of mass quantities and resultant production of "excess" nitrate...
The "goal" is to keep the deitrus "suspended" oxygenated and available to be "broken down" by micro flora and fauna (different than "decomposing in a dark corner) and allow it to be removed ala skimmer and regular WCs as DOCs