It all depends on what size tank you have first of all, and of course the order or which you put the fish in your tank. My suggestion to you is if you do in fact have a big enough tank to house 2 tangs go ahead and add the purple as long as you don't plan on adding any fish after him. There is always the possibility that even though he was the last fish added, he could still harass the existing fish, but that's a chance that anyone takes when adding anything to the tank that is on the aggressive side. I have a 125 gallon and had a purple tang for around 5 months by himself with just small fish. I finally got a hippo around the same size and added the hippo. The purple never attacked unprovoken. The hippo intagonized him, and the purple would swat the hippo with his tail. But never he actually chased the hippo. When I added the sohal a little after the hippo, the purple tang went CRAZY and I mean CRAZY on the sohal and the sohal is twice the size of the purple(7-8"). But the next day, the sohal was the king of the tank and still is. Like I said, tank size DOES matter. Good luck with your decision! I love my purple, and like you said, everyone has a yellow tang!
Melly
