I disagree with your LFS about the tang growing slowly. I got one about 2 months ago @ 2" and he is already 3"!!!!
Mine, at least, is growing like a weed in my yard! I better hurry up and get my new 150 gal going (stupid lighting-hell quandry slowing me down) so he has room to swim around in

He's ok for now in the 37gal, but I fear it won't be long before he's outgrown it.
Just my $0.02