Even small pieces of live rock contain anaerobic areas. It is becuase the rock is so porous that it can function as both an aerobic and an anaerobic filter, aerobic conditions exsist on the surface and larger pores, anaerobic conditions exists within the smaller pores. Its the porousity of the live rock that makes it a better coice over bio-balls which are for the most part, non pourus and will just build up detritus over time.