I peaked at Amazon, but want to give my local shop a shot before I go online, but I refuse to go into a PetCo or PetSmart, they are gougers in this area.... I will have to take a peak and see how much those are... I just have carbon in mine right now, but rubble rock is that a specific type of media or slang for something else, because I could put that on my top tray, then have carbon on the middle and keep the bottom open... hmmm ;)