My foxface is easily 6" right now and eats a full nori sheet in a matter of a few hours just by picking at it randomly.
My Yellow Eye Kole Tang and my Hippo Tang might touch the nori once a week and only when I soak it in garlic first.
I had a yellow tang and he was small but his favorite thing was race from side to side in the tank. I would not give them less than 6ft of tank space just because of how fast they are.
Edit: Should also mention my Hippo Tang was bought about 2 months ago around 1" long and today shes almost 4" they grow FAST
35gal Reef, 20gal Sump--2 years, 3 months--Shut Down
If you can't fix it with a hammer, you have an electrical problem!