you can do a fish-only tank without live rock, but, because of the filtration live rock does , you would have to do more water testing and changes since it could mean higher level in ammonia, nitrites, and nitrates. Also, it might attract visitors more in your house because the tank has a more natural apperance. So, if you don't want to include any live rock, stock your tank with a few small, extremely hardy fish, or a specimen tank of a larger fish to feep the water quality high.