Yeah, you can do that, live rock isn't an absolute necessity, but fish do like places to hide and sleep. The BB will grow on the sand, and whatever decorations you put in there, an in your filter media. Just make sure you have enough (and large enough) things that the fish can hide at times. When fish get stressed, they hide for a while. They also hide away to sleep, cuz in the wild, if you're sleeping out in the open, then you're gonna get eaten. If they can't hide, they'll get stressed, and stressed fish will get sick and maybe die. But there are fish-only tanks with no live rock. There are no absolute rules to aquariums; decorate it how you like. There are some beautiful natural saltwater plants that you could put In There, too that don't cost that much. Check out live-plants.com. Their shipping rates aren't bad, either.