rehoming can be more difficult than you're probably anticipating.
for example, i have a 160 gallon tank, that could easily accommodate your tang. however, i already have a tang of my own that don't take too kindly to other surgeonfish. so i wouldn't be able to accept him as a rehome.
that's likely going to be a common story. once you start getting in to people with truly large tanks, most of us have already spent/are spending countless hours pouring over potential stock lists and making plans. i can tell you right now that i have my entire stock list selected for my tank, i'm just waiting to find the right fish, right amount of free time for QT, and the right amount of disposable income to complete it.
so then this puts you in the position of taking possibly having to take it back to the fish store. i don't like doing that. odds are that the fish will just end up in another entirely unsuitable tank. i've had to return a fish before, a six line wrasse which was a bad newbie purchase. now i know better, but it's not a good feeling. when i get a fish i fully intend to keep it happy and healthy for the duration of its natural life, just like i would with a dog or a kitten. in my mind pets are pets, they're part of the family, so swapping them out at your convenience isn't a good strategy, and creates problems for an industry that is already hard on natural resources.
you also won't get to truly enjoy the fish in a smaller tank. when i started i set up my 75 and got a yellow tang, another ill advised newbie purchase, and he was never very happy in that tank. quite often irritated and exhibiting compulsive behavior like pacing or fast back/forth swimming.
after upgrading to my 160 there is a world of difference in how he acts. much more relaxed/swooping style of swimming, and considerably less aggressive towards other fish.
there are a lot of really gorgeous fish you can get for that size tank, i would recommend focusing on size appropriate options, i think both you and your fish will be rewarded in many ways.
-edit-
in re-reading your original post you also seem to have some confusion regarding ich, i would suggest checking out some of the stickies in the fish disease forum, starting with this one:
http://reefcentral.com/forums/showthread.php?t=2159738
misconceptions about Cryptocaryon irritans abound in this hobby, but there's excellent information available to dispel them.