You can start by researching livestock for your tank and make the appropriate stocking choices based on the size of your tank. Putting a fish in a tank smaller than it should be in will contribute to a fishes stress, which makes it more likely for a parasite to take hold and infect the fish
Since you didn't bother doing that here, you're only option is to treat the fish if it is unable to fight this off on its own. You're going to be best served by doing this in a seperate QT tank, as anything that will kill the parasite is not safe for reef tanks. You might as well take all the fish out of the display and treat them, and leave the tank fallow (fishless) for at least 8 weeks