Do i treat tank for ich?

So I purchased a sailfin tang two days ago and this morning his color was gone and I noticed spots but it was to faint to tell if it was ick yet or not. when I get home it is much worse and clearly ich so I went out to get a small qt to put him in. Right when I got back with the tank I watched him belly-up:bawling: my question is should I treat the tank for ick? all the other fish are fine and very healthy