I probably wouldn't risk another tang. If you have to get one make sure it's from the species of tangs . "Example hippo's and yellows get along most of the time while yellows and scopas will have WWWIII and I can't remember the species names to save my life." You might want to either add more snails or maybe a lawn mower blenny," keep an eye on it though some of them are a bit nippy." What are your water parameters," do you use ro water", and how long has your tank been set up? Excess nutrients are usually the culprit though if you have enough snails etc in the tank .
hth
Chris
*edit sorry accidently used a French word"