Did you get an adult, or juvinile? And what other fish do you already have in the tank. I agree emperor are pretty tough, once they get established though they will kick everyone's behind. So this may be (one of) the last fishes you add to the tank. As always, just make sure you go through a thorough quarantine- a week or so to make sure it eats like a pig, then 3-4 weeks of medication to rid any parasite. I've had my since he was a two incher and he's now a beautiful six inch adult, eats everything I drop in the tank. If you already have other fish at the time of introduction, try moving the rock work to blur their existing territory and allow the angel a chance to find a shelter to claim as its home, this will help diffuse turf wars and help it settle in faster.