Fish won't die just because you don't have live rock. Live rock does provide good surface area for nitrifying bacteria to grow on and can be an excellent biological filter. It also looks really nice, but you don't have to have it for a fish only tank. With a 38 gallon, you could get clowns, gobies, just about any damsel (but they're mean), wrasses, basslets...check this site's livestock section and some others on the web..most will tell you how big the fish gets and what its tank size requirements etc. are.