to be honest, if your going to do a 5 gallon, might as well make it reef. you could get some really neat corals and such, but fish wise, you can really only have one, for example, a baby fire fish, green chromis, nano gobbys, or blue-tail damsels. but if you do decide to get a fish get them as babies so they wont get to big for the tank. Also get a rather large filter for the size, maybe a 20 gallon filtration system to keep the water as clean as possible, and do plenty of water changes! I hope this helped & if you have anymore questions that I didn't cover just let me know, and I'll try my best to help!