They won't get to their full size in a small confinement, in fact they won't live a percentage of their real life span. They are continuous feeders, so in a large reef, with plenty of food, they will get to full size. Don't buy any tang unless you have the proper room for it. They are getting fewer and fewer on the Hawaiian reefs, so we as reef owners have to be serious about at least trying our best to keep our live stock alive and happy. I get sad when I see a yellow tang in even a 55 gallon tank. I have three of them, they are over 4 years old and the biggest one is 5" I feed daily and they browse constantly. Some people feed a lot more, and as a result, have bigger Tangs.