Also how long did you let your tank cycle for?
You need to setup a hospital tank and your display tank (DT) will have to remain fallow (free of fish) for 72 days. There are a couple of treatment remedies to treat Ich once in the hospital tank. I'd suggest reading how to get the hospital tank up and running, moving the fish, and then address treating it.
How big is your display tank?
The PETCO $1/gallon sale is going on in most areas. You can pick up a 10-20g tank on the cheap.
You may also consider returning the tang. You'd still have to run fallow for 72 days.
Good luck
The 55g tank isn't big enough for the yellow tang. You should see if the store will take him back.
The tank may not have cycled properly and probably wasn't ready for fish. Most people say it takes about 2 months.
What do your parameters tell you.
Not sure you'll be able to get far in this hobby without a QT or HT protocol. It doesn't take much room. A 10g tank should work and can probably fit on a kitchen counter if there is no where else. As Tony (Fishin) said the ich will stay in your tank even if you manage it and don't see symptoms.