A ten or twenty gallon tank does not make a good QT tank in my opinion. I know people buying a 3" tang and stick it in a 10/g tank. I don't think even a 40/g breeder is a decent QT set up for large fish such as tangs/angels, probably ok for smaller fish.
I do not believe in medicating any fish. In the last 15 years had only one incident with velvet which is not that bad statistically speaking.
Here's how I quarantine:
-First, I observe the fish at the LFS to make sure it's active before bringing it home, also observe tank mates to make sure no sign of ich or any other issues.
-Remember some LFS keep their salinity very low (around 1.015-1.019) to keep the fish parasite free so acclimate fish for at least an hour minimum.
-My QT is very different (not really a QT but works for me). I have a 50/g lightless refugium with few pieces of LR under the stand which is connected to the main tank. I keep the fish in their for at least two weeks. I have a small led fixture I turn on in the evening to feed the fish. Basically fish stays in isolation for few weeks before going in the main tank. I have done that with all my angel fish and so far no issues.
-Based on my experience,
ICH is caused 99% of the time due to fish being stressed; improper handling. I don't need scientific proof to know that.