I'm kinda concerned that you have so much in a 3 week old tank. How big is the tank? Water parameters?
If it is in fact ich you will really need that hospital tank up and running. It can be a simple setup with just a filter or water flow and heater. No sand or rock in there at all and maybe a pvc
pipe for the fish to hide in and feel safe.
Your two choices to absolutely get rid of ich are hyposalinity and copper which are the only two methods of killing it off without worry. Those chemicals your lfs
may try and sell you may work for a time but the ich will live on in the tank just waiting to attack another fish.
As a side note, blue tangs and most tangs for that matter are ich magnets and it isnt rare at all to see ich surface on them when they enter a new tank. You said you didnt notice it when you bought it, could it have been there all along in the store?