I know that you can use the plastic plugs, the man-made plugs, rubble and various man made disks..but this is soooo simple. My lFS told me just to buy (funny how they just had one on hand) a diamond blade for a dremmel ($10) and then take one of my pieces/or buy a small piece of tonga and cut it into disks. It is great...and I have to say that a small piece of tonga in my tank is enough for me so this constitutes "reef recycling"