live rock rubble is better than bioballs since the live rock contains micro fauna which will break down other substances besides ammonia and nitrites, the micro fauna breaks down nitrates phosphates and some other stuff as well as breaking down ammonia and nitrite. Bio balls clean 2 things, live rock rubble cleans 4+ things. Hope this could clear some things up. Dont buy some live rock and smash it up, try to ask the lfs to give you already rubbled rubble. My lfs live rock area has all these pebbled size rocks at the bottom and im sure your lfs could pick those out for you.