Wow 38 gallons and two blue tangs. I have been on the fence for 4 months about adding another tang to my 6' 125 that already has a 4" scopas and a 4" yellow tang in it. They don't really use up all the 6' since I only have about 4' of actual rock work to allow swimming room, but they do use up all the 4' that they prowl in. They really can accelerate fast in short distances and the more room they have the happier they will be. The whole I'm giving them a better home than someone else isn't really valid in this case seeing the size tank they are in now. Sorry if I sound rude, I've been told recently that if I wanted to add another tang like a Blue tang or a Powder Blue which is what I really want I'd need to part with at least one of mine. Responsible reefkeeping is what we try to preach to everyone on this site, and that's all these people including myself are trying to tell you.
But to tell you the minimum size tank that a Zebrasoma Tang would need would be at least a 75-90g tank and a 4'x2' 120g would be ideal for a zebrasoma tang. With a 4'x2' 120g you could get away with two Zebrasoma's like mine. With a 6' tank like a 125g your options do open up as long as the fish doesn't get over 9-10". Tangs like to make what I call very wide turns at speed so they need to be able to maneuver with plenty of room to make the turn.
