a while back,I purchased a clownfish from the LFS and didn't QT it,the next day after adding it in my tank,I noticed what I thought was ich(still not sure what it was)after he found my nem,he cleared up but my PB tang started getting spots on him,he fought it off as well without QT or any meds......I do add Omega 3 fish oil to flake/pellet food which helps keep their immune system up,if you don't already do that,it would be worth a try...my PB tang kept spots for several weeks but it never seemed to bother him and eventually,it was gone...none of the other fish had any spots,but they have all been in my tank much longer that the PB tang too
if I have already suggested this in a previous post,pay no mind to this post,I just woke up