fish size

Hey yall, I have a 1.5yr old tank and sailfin tang. this is a 37Gal tank with more than approp filtration and LEDs. The sailfin is getting secretive and big. Whats the judgment on when to send him elsewhere and replace with smaller. bummer as he/she is starting to change patters/colors so...

Just a quick Question, Off with the missus to get our first fish on Friday. Last time we were there looking through the tanks, we noticed quite a big difference in size of each fish on display, Ranging i would guess from their new stock of younger, smaller fish, to the moderately sized bunch...