How long has the tank been set up? Certain fishes such as dwarf angels and mandarins should be added to a well established tank. Dwarf angels like to graze on algae, so if there should be an established algae population. Mandarins primarily eat small organisms such as copepods that live in and around the live rock and sand, so you need large enough populations of those to support a mandarin. It sounds like you may be new to the hobby (correct me if I am wrong), so you probably want some easy to maintain, hardy fish. Good examples would be a Royal Gramma, various gobies and blennies, firefish, ocellaris clowns, skunk clowns, yellow tang, chromises, or as previously mentioned a reef safe wrasse like the 6 line. I am probably leaving a bunch of other suitable choices out, but this gives you an idea. Research any fish before you get it. You want to make sure it will not outgrow your tank, doesn't have specific needs like mandarins, and is reef safe. Most angels, along with triggers and a lot of wrasses will eat your corals. Some fish, like hawkfish, will eat your shrimp (assuming you have shrimp). But don't worry, there are still plenty of attractive choices such as the ones I posted. Good luck!