BHT's have conflicting info. Some say they are ich-prone. Others say it is hardy.
My experience, I hypo-ed the guy after a serious ich attack. That was a year ago. After that, he hasn't had any problem since. Well....I haven't added any other fish in there but a maroon clown eversince the hypo finished...I guess both are true then, you know?
BHT's are prone to ich so, among other fish, the BHT would most likely be the #1 victim when ich starts in your tank. So QT is the best way to prevent that.
On the other hand, I've had my BHT for a year now and it is plump, smooth, eating very well, never had a sign of sickness, and is very happy. I added a maroon that looked healthy. I risked the BHT by not qt'ing the maroon. Thank god I was lucky, nothing happened. I've been adding corals to my tank eversince and voila no signs of stress.
As a matter of fact, when I only had the BHT (this is before the maroon purchase), I bought a yellow tang. The yellow tang was alright. Then, it turned pale and pinkish in the face area. Then it died. But the BHT was happy, swimming about, and had no incidents of disease or parasite. So I guess it is, TOO, hardy.
We live and we learn