Geez, I was in this position a couple of years ago. Stupidly bought a powder blue on sale 9.99 from a London shop - yep, big ich problem, very sick fish, didn't look so at the store. I came to the group, did some research on different medications, treatment protocols and made an educated decision. Should have known better and quarantined the tang, but didn't. Lesson learned. Set up a quarantine tank moved all the fish to a quarantine tank and treated the quarantine tank. Left the display tank empty of fish and running for over a month to let the ich run it's cycle and die and made positively sure that all ich was gone before returning all of them to the display, lost the powder blue ( found out after 2 others bought the same fish, same results - ich - dead tang) I was the only one who acquired ich out of it. Learned to buy from reliable sources, no too good to be true sales any more.
Brought another tang home a few weeks ago, yep, he showed ich from stress and decided this time around to leave well enough alone and soaked food in Kent Garlic Xtreme, wait it out and see if the fish could recover without intervention and within 2 weeks all signs of ich are gone. I was concerned that if I moved and treated that I'd only stress the fish again returning it to the display tank after treatment. I'm watching the tank like a hawk for any issues, but so far so good, no other signs of ich all are healthy and eating like the pigs they are.
What I learned was research and be prepared to do what needs to be done in order to treat. I have a tank sitting here just for quarantine purposes. I use it for mixing my salt water too.
I prefer to deal now with people I trust and they are out there, I have 2 go to stores, Sealife, cause they're like family and Big Al's London and I like private sales too. I haven't met Aaron, one day I will, they're good at the store, but we need to research and make an educated decision. I blame myself for the whole scenario. I knew that tangs were very high risk for stress and ich but didn't quarantine. I am responsible for making the disaster decision.
Best of luck, lessons learned.