How much rock you will need depends on how dense the rock is and how big you want your scape to be. Generally, I would say 20lbs. +- a pound or two for that sized tank. I would go for the base rock and just seed it with a few pound of live rock, or rubble. You may want to check the tanks at PetCo and take a look at their rock before you make your decision. Some stores take care of their tanks better than others, so it never hurts to check. Look for hitchhikers, click on the link in my signature to get an idea of what to look for. If you don't have a mobile device with a browser, you can print it and take it with you if you feel like you may need a reminder. Hope I helped.