are you sure he's thriving? because they do tend to get stunted growth in smaller tanks... if he was smaller in size when you got him, (2-3inches) he should be after a year and a half about 4-6 inches.
he's already about 4-5 inches...he's gotten bigger since when i got him. he eats great...always has good color...and gets along with everyone else in there. he even cleans the tank a little by eating the algae
You do have a larger tank for him when he starts to get to his adult size right??? If not, it doesn't look good for him in the long run. He needs around 125 gallons as posted above by hail_sniper. I have my blue tang in a 20 gallon but she's only the size of a quarter and I'll be getting a nice 250-300 gallon for her and her future tankmates includeing a yellow tang, christmas wrasse, keyhole angelfish, achilles tang, fire goby, royal gramma, and true percula clownfish, and I still think thats pushing the limits for those three tangs. They need a lot of room to swim... they maybe only 4-5 inches but they need 4-5 feet to roam and swim freely.