Sorry to here about your tang. Here is the board to do your research and get lots of help:
https://forums.saltwaterfish.com/t/127007/faqs-fish-diseases-treatments-quarantine-health-info
It is also very important to provide the best possible envirment ant food for the fish for it to pull through. Please give all you specs. so anyone that wants to help knows the full story.
Tank size-
Age-
Filtration-
gph-
How long you have had the fish-
temp-
salinity-
amm.-
nitrites-
nitrates-
And anything else you test for-
How you maintain your tank(water changes, amount,how often, type of water, ETC.)-
What stock you have-
what and how often you feed-
This will help to try and see where the problem is and what can be done to help. You can't just treat one fish for ich, so you'll need to decide if you are going to treat all or try and improve and hope for the best.
You will need to makle a well educated decision. So do your research.
Pattie