says that blue tangs take around 8 years to reach 12" and they rarely have anyone take them back that have gotten that big. And if yours did get that big I'm sure most local fish stores would love to take a large blue tang because they probably know someone with a monster tank that doesn't want to wait 5 yrs to have a larger fish in it.
I'm currently setting up a 135g as well. I have a small scopus tang I'm going to put in it first. I love this fish, it colored up really nicely. I kinda thought it was kinda ugly at first but now it's one of my fav fish. It has flecks of blue , and fades from yellow to brown front to back.