Your saying you want to use the granite as a background. For my tank that would be 6' long 1/2" thick and 38" high. That's a LOT of weight to stand on edge and put in the bAck of your tank. Maybe your tank is smaller but then the glass your tank is made out of would be thinner. I am a firm believe in weight displacement with tank decor, suck as live rock, hokey rock, river rock, etc. I don't think this is a great idea regarding the weight.
Mike