That really depends on a lot of things. How big is your tank? How long is it established? Water parameters and how long fully cycled? How much live rock? How much water circulation? Other filtration? Do you have corals (some fish will eat them)? What fish and other inhabitants are in there now (some fish might eat them)? Some fish are peaceful and others are aggressive. You have to find out which ones are compatable. What kinds of fish do you like? Do you have an idea about how many fish you want to have? Some fish can't stand their own kind while others do well in groups. Sounds like a lot, but you have to plan this out. Generally, you should add fish to your tank slowly so as not to overload the biological system. Give the biological system a chance to adjust to the # and size of fish. It's also best to add the most peaceful fish first and the most aggressive last. I would also highly recommend that you quarantine new fish before you add them to your main tank. Make sure only healthy fish enter your main tank. Read everything you can on this website, talk to your local fish store staff, read books on the hobby and ask questions on this site. A good fish guide is Marine Fishes by Scott W. Michael. It gives some quick reference info on over 500 fish species. If you answer some of the questions I just asked you, I'm sure you will get a lot of helpful answers. I hope this helps. Good luck