when i got ich, it was when i first got my tank. I bought it as a complete reef setup, and one of the fish the lady had in there, a kole tang, had ich. so i treated with copper in a qt, and it didnt work. well, i didnt want to put them in that tiny tank again. so i tried the garlic, shrimp, and kick ich combo and it worked.
now, i siad i have never had any problems with buying a fish from the store and putting it directly into the tank ever since