Thanks Doug!
Mitch - sorry, the "rocks" in this case being zeolites. The zeovit system is so named since you put these zeolites in a reactor and I guess they're porous and thus create new surface area for the dosed bacteria to colonize (I'm sure some settles on the regular substrata as well). The instructions tell you to change it out every 6 to 8 weeks or thereabouts.
http://www.jlaquatics.com/product/zv...-+1+Litre.html
I don't think Prodibio makes use of zeolites but I believe both the UltraFauna and Brightwell systems do.