Welcome to R2R! I wouldn't suggest a blue tang in that size tank. While I am an advocate for keeping tangs in slightly smaller tanks than is often recommended as long as their other needs are met, hippo tangs and other open-water species are an exception. Think of them as little tuna, they very rarely stop moving in the wild. Zebrasoma and Ctenochaetus spp. often spend time hovering in the water or slowly cruising, and as such can be kept in smaller tanks than other tangs (eg. 3-4'). However, the reputation that tangs get as hyperactive little monsters is fairly well deserved for the other species. Even a 3" hippo would need a 3-4' foot tank to stay healthy, and an adult needs at least a 5'x2' footprint and lots of flow to get enough exercise. Tangs are also rather difficult to care for, for a few reasons. They can be really mean, they need frequent feedings of algae-based food, and they can get diseases super easily. Instead, I'd go with an azure/allen's damselfish. They are tough as nails, do well in tanks as small as 5 gallons, and shouldn't be bullies to other fish. Other species of damsels can be mean, so make sure that you are getting the right one. Good luck with your tank!