125g is plenty of room for them ( Reefescape has one nice one in a 50g i believe) , they only get over a foot long the ocean i believe , and they don't really swim much as they are weak swimmer. I have a 4-5 inch in my 25g shallow and he seems to do fine ..for now , he even spit water at you when he see you coming with his food so the kids will love that. Just make sure he's the first fish in the tank and don't house him with any aggressive fish because they are slow swimmer, also moderate flow too , not too strong. Also buying one thats fairly decent size (3-5 inch) will significantly increase the survival rate ( I found this out the hard way..lol)