The condition you've described is commonly referred to as pop eye. Pop eye is most often associated with trauma or severe stress. Moving your fish from one tank to the other, for instance, is stressful. Your Purple Tang may have effected trauma on your angelfish during one of their past engagements, or the trauma could have been self-induced by it accidently scratching it's eye against the rocks (maybe in flight of the Tang). Rapid changes in chemistry (especially pH) can also produce this type of condition, although you mentioned you acclimated him slowly.
If trauma or stress are the culprits (and they usually are), the fish will cure itself in a few weeks once it's become comfortable with its surroundings. Feed it well, and guard it from further stress. Remove the Tang if need be until the injury has healed itself.
There is the rarer possibility of a bacterial infection. In this case, you should remove the fish into a qt tank and administer a wide array of antibiotics. A combination of saltwater myacin 1 and 2 (gram positive and negative) would be my first prescription. Because infection is far less common, I recommend you perform medication only after you've given time for your angel to heal on its own first. If it problem persists after 2 weeks (without stress, that is), or gets progressively worse each day, it may be bacterial.
[ November 14, 2001: Message edited by: Leonard v2.01b ]</p>
