I am confused about this notion of a fish magically acquiring ich through stress. Where are we purporting that this ich comes from? If a fish is properly quarantined and treated before introduction to the final environment, there should be no problem with ich.
Unless the ich already exist in your main tank, which is a different issue altogether...
EDIT: Also just some simple math. Lets assume a yellow tang is 2" long when purchased as a juvenile, and has a generally documented adult size of 6-8" according to various semi-reliable sources on the internet. The recommend tank for this adult, 6-8" fish is 6ft long - 72". An 8" fish in a 72" tank is 1/9th the total length of the tank - 1/3rd the width and height if it is perhaps 72x24x24. That would be numerically the equivalent of housing a 2" individual in an 18" long tank that is 6" wide and tall. Whoa whoa whoa. I'm not master of physics and architecture, but I believe 18" is a fair bit smaller than 72". Hmm, it appears to be precisely around 1/4th, just as 2" is 1/4th of 8"....
Moving along...
What then, perhaps, when this fish grows to 4"? This magical 6ft rule (for an adult, 8" fish) seems to imply that a 3ft (36") tank is perfectly suitable for a 4" fish. How long does it take the average yellow tang to grow from 2" to 4" in captivity? I am not sure of this figure but I imagine it is at least longer than 24-48 hours.
So what does this say about a 48" tank? 48" is roughly (and by roughly I mean exactly) 2/3 the total length of a 6ft, 72" tank. So theoretically according to this magical fish-to-tank size ration, a 48" tank would be exactly capable of containing a yellow tang until it reaches the size of 5.34".
How long does it take this 2" tang to grow to 5.34"? Seeing as how it takes at least 48 hours to reach 4", I assume this 2" tang would have at least a weeks time in the 48" tank before it reaches the magical 1/9th tank length size that renders it instantly susceptible to depression and inherent death.
Now I will certainly be the first to admit there are those who know better than I, but if someone can please explain to me, in very concrete, verifiable, repeatable terms that a tang of ANY size is only capable of surviving happily in a tank of at least 72" in total length, regardless of proportion to the size of the fish, I will be more than happy to join the club of those who bring down the Tang hammer and hold up the 6 (foot) Commandments of inhumane tang opression.