Then maybe add a rock or two
Either one could work. I have no science here, just logic, but sump would allow him to "forget" about his previous territory which is good, but the acclimation box could help him get accustomed to the new fish, which is also good. Dealer's choice from my perspective, I can say I've had luck with the in-tank box though. Not sure how big your tang is, but you also don't want to jam a giant tang in a tiny box, so if that's the case maybe sump.
Gotta be in the display or it’s pointless.. you can just put the yellow tang in time out in the sump if you want but most likely won’t change his tude.. it won’t be long and that powder blue will most likely destroy your yellow tang lol..
Powder blues are notorious punks and will run your tank in no time.. do you plan on adding other tangs at any point? If so now would be the time and let them pick the pecking order and distribute the aggression amongst each other… I’d say once that powder is established it’s gonna be hard to add any tangs or even other fish. Not always the case but mostly is with a powder… most people recommend the powder to be the last fish in the tank.
How big was the mirror? It needs to be a decent size hand mirror thats larger than the fish so they can see all of the body. You can try a second mirror. I have had to use two mirrors before. You might be better off moving the existing tang to the sump or another tank for a week or so and then try moving back with old tang in an acclimation box for a week or so.
