Yes and No...
If you want to do it "correctly"... You will need to allow the tank to go through fallow.. So you would need to pull all your fish, and quarentine them for 6-8wks on average... Depending on the stage of life, ich can live for around 40-60 days without host(on average, though some reports show as much as 100+ days)... Since there is no cost effective practical way of testing if ich is still present in your tank we use the 6-8wk rule as a general basis under the assumption that ich most likely died after this time away from host(though it is possible for the ich to die much sooner, or even still be present after the 8wk mark)... Now while your tank is in fallow, you will need to tend to your quarentine of fish... Which you could now treat with Copper and or hypo... So their recovery should be much speedier than the 6-8wk mark... However you cant put them back in the tank until after follow, so you must continue with the routine waterchanges and such to keep them alive, not to mention running the quarentine tanks plus your normal tank....
In a nut shell that is the "correct" way... HOWEVER... there are no garuntees that you dont go through all this and your fish still end up with ich after fallow(though its a low possibility)...
On the other end of the spectrum
99% of the fish that you have in that tank are quite hardy and or resistant to ich. My suggestion would be to just monitor the tang, keep him low in stress, and feed well and often... Supplement garlic for all inhabitants... You wont get rid of the ich, but with time and the right conditions the fish's immune system should be able to do the "heavy" lifting for you....
Now you could pull the tang and only the tang and quarentine him(more like hospitalize)... BUT... My concern here is... Stress from a move like that could worsen his condition... and a move back into the tank could just cause stress and bring back the already present ich.... But a freshwater dip and or hypo quarentine may just be what he needs.
You have to make the choice about how serious you want to take it... Ich is quite common, and most fish can supress it, however these tangs will always come up here and there with it.