Lots of odd info on this thread so far...
All tanks do not always have Ich. All Tangs (Hippo or otherwise) do not always get/have Ich. However, some fish can have Ich, usually in the gils, where you do not see it and they do not display the "white spots" usually associated with Ich. UV
filters in no way guarantee that Ich will not be spread to other fish. It kills what is in the water column, but that only gets some of it and only in certain stages of the life cycle. There are two views on dealing with stuff like this...
(1) Maintain pristine water conditions and healthy, well-fed fish. They might be able to fight it off/ keep it at bay. People on here have gone that route and never had an Ich outbreak. IMO
, that amounts to a bit of luck. Ich is not always a death sentence. Some fish muscle through it and can eventually overcome it, but some can't.
(2) Treat fish with either copper or hypo in a QT and leave the DT fallow for 6-8 weeks. IMO
, that gives you an Ich-free tank and fish going back in are Ich-free. No more Ich. If you QT future additions and treat as needed, you maintain a "clean" DT. Is there a chance that a fish with Ich sneaks through the initial QT "observation period" and brings Ich into your DT? Sure, but it is not very likely. The down side of the QT while treating is that hypo or copper will likely stress your fish. Some might not make it. Again, that end result probably has a bit of luck with it, but most fish do okay. I like hypo better, as copper is basically a poison and seems to stress fish more. Both have worked for many, many people.