First of all don't panic and just start dumping stuff in the tank to cure it. The Tangs didn't get that way overnight, and you're not going to cure them overnight. If possible I would definitely put them in a hospital tank.
Why don't you give us a run-down of your equipment, tank, lights, foods, a history of what you've done so far (i.e was the tank moved?)...everything.... and we'll walk you through it while you're researching. Believe me everybody here is more than willing to help, and we definitely don't want your fish to die.