heya... I just have a question. We recently got one of these protein skimmers Bakpak CPR-BAKPAK Premium Aquatics and since it was used I have no manual or idea how to use it. Well, I figured out the main parts, but friends of mine have told me that you need to place rocks into the long open square chamber in the back. Otherwise, it seems to work fine. If I have to fill it and we don't have much money, are there any safe substitutes for whatever you put in the long square chamber of the Bakpac protein skimmer? Thanks! =)