If the fish has ich you have two choices.
Take your chances on a hope and cope strategy with low stress, good nutrition perhaps with garlic,uv sterlilization, cleaner shrimp and other things that may or may not help but won't cure it. And don't introduce any new fish into your system for 11months which is the time it will take for a strain to expire after a maximum of 34 reproductiuon cycles.Even if your fish survives it is most likely that it has a developed only a partial resistance and the ich will still survive in the tank unseen usually in the gills. Any new fish introduced paricularly a tang.even a properly quarantined specimen,will likely spark a new outbreak
Remove all fish in the tank and treat with copper,hyposalinity or the tank transfer method leaving the main display fishless for several months,
Good Luck.