I think everyone has covered your options as far as treating the fish and keeping them out of the display fr 8012 weeks vs living with ick. I will add that I also have nevr quaranitned fish ging into my tanks either and very rarely ever see ick and that ends to be just after a new fish is added or something and stress goes up.
What I will add is that one thing you can ty if the powder brown is getting bullied and you want to try to keep him is to re-arrange the rock work a bit. This helps to get rid of territorial issues for fish that are already established and sometimes can do the trick.
Lastly I would make sure you are feeding a good amount of algae/greens to the tangs regularly. Mine get nori, lettuce, formula2 and garlic soaked mysis daily.
What I will add is that one thing you can ty if the powder brown is getting bullied and you want to try to keep him is to re-arrange the rock work a bit. This helps to get rid of territorial issues for fish that are already established and sometimes can do the trick.
Lastly I would make sure you are feeding a good amount of algae/greens to the tangs regularly. Mine get nori, lettuce, formula2 and garlic soaked mysis daily.