I think you are on the right track, the new tank will restart the bristletooth's territory and both tangs should be on an even playing field especially in a 180. Make sure you don't under size your powder blue compared to your bristletooth and buy the chubbiest powder blue u can find. I think you'll have no problems. When I add a fish I use this stuff called reef keeper a day before and for the next 4 days after adding the new guy. I don't believe in the chemical stuff much at all but I did see this stuff work on a purple tang but it's basically some type of immune booster, reef safe, and your skimmer takes it out. When I got this little purple tang it got ick about 3 days into being in my tank, he ate great and didn't really scratch much. In the mornings he had a few spots but by the afternoon he was covered pretty well. This lasted for two weeks until I went into a LFS which had a purple tang in their display and I started talking about my tang. He told me to try reef keeper and I told him none of that stuff works, no thanks. H said," buy it,if it doesn't work bring the empty bottle back and I'll give you your money back." I still almost didn't do it but I did. I got home, my purple had some spots so I dosed strait into the tank and left. I came home 6 hours later and this fish was clear, I know BS but I swear! Maybe it was pure luck but I went to the shop I got the purple from and told him. He has been carrying it for over a year and sells it of the shelfs. I think if the fish is strong but has ick like my tang the Reef Keeper might just help but I don't think it's an Ick killer. So I use it as a booster when needed. Sorry to ramble but just sharing my experience, if anyone uses the stuff especially having tangs let me know.
