Here's some info right off the
http://www.marinedepotlive.com/ website:
Description
Purple Tang - Zebrasoma xanthurusAlso known as: Yellowtail, Purple Sailfin Tang
Maximum Size: the Zebrasoma xanthurus grows up to 10 inches.
General Size Specifications: The small size will come to you generally 1 to 2 inches; the medium generally 3 to 4 inches; the large generally 5 to 6 inches.
Minimum Tank Size: The Purple Tang prefers a tank of at least 100 gallons with plenty of places to hide & swim.
Diet: The Zebrasoma xanthurus is a herbivore and likes to eat Dried Seaweed (best), marine algae, mysid shrimp, Spirulina, Romaine Lettuce (blanched) , fresh seaweed, and other meaty treats.
Level of Care: The Purple Tang is a medium maintenance fish.
Behavior: The Purple Tang may act peacefully toward other fish.
Hardiness: This is a hardy fish.
Water Conditions: Keep water quality high (SG 1.020 - 1.025, pH 8.1 - 8.4, Temp. 72 - 78° F).
Range: the Red Sea.
General Notes: Once rare and almost impossible to import, the Zebrasoma xanthurum or Purple Red Sea Tang is a frequently sought member for the aquarium. The most aggressive an belligerent member of the Zebrasomas, the Purple Tang will likely pick a fight with any member of the Surgeonfish/Tang family as well as any fish that looks similar to a tang. Should be the last fish and best kept as the only tang introduced to the tank unless housed in a very large aquarium with other aggressive and large tangs. Must be fed vitamin suppliments and proper herbivorous diet to retain vivid color and prevent head and lateral line erosion (HLLE)
Long and the short of it is you have too small of a tank and too many fish for the Tang to ever be happy.
Sorry
