I'm told by both my LFS's that the Tropical Marine Centre have their own very intense quarantine procedures before the fish even get to the shop. That's what i've been told anyway, and I can only go on that and trust. I spend a lot with them and they always tell me if I want something and its not ready or suitable.
And I don't run a QT and nor will I. Not because I don't have the time or resources, I have both, but because Ive read a lot about fish immunity on here and elsewhere, including one of the oldest tanks around, and I agree that they should be able to build up resistance to the diseases they are likely to encounter rather than being put in a sterile, isolated tank, stressed out and then treated with who knows what just in case us humans think they 'might' have something. And then put them in the tank and they die because they cant fight off disease of some sort.
If I ever have an issue, I wouldn't hesitate to use one of the modern reef safe medications. Its 2017/18 not 1970 and if you read the reviews they will generally give you an idea of effectiveness, or not and then you can make an informed judgement.
I read up a lot on diseases and in addition to constantly running a UV-C, I also feed the fish at least every other day with Gamma Frozen cubes which include a garlic supplements which has been shown to potentially kill parasites including marine ich and you can read up on all of this.
Ive been advised to give them a well fed and healthy, mixed diet, and I give them the best i can as well as keeping their environment were it should be and that's what ive done.
I also have the JBL Pro Cristal - 36W UV-C (aka The Rocket Launcher) same as
@Nobbygas (because he likes to copy me! lol) but you can probably get away with a smaller version or different type. Go as high as you can on the W that will fit somewhere in the tank because the more W, the more killing power. And run it 24/7.