Having had a ich break out in m system I know the stress of it. I have come to not being afraid of ich on my fish. It comes and goes I just make sure the are fat and healthy. I also qt all new fish going into m system. I know there is ich but im more afraid of other stuff. Also I dont have a blue tang but seems s though they are magnets to ich. You look at them wrong and they get it. As others have said mabe find a new home for the blue tang might relieve some stress.