My yellow tang used to be very active. About a week ago I introduced a new fish, a mandarin. Since then it seems the tank's harmony has been altered and my yellow tang is hiding under a rock, not feeding. He is one of the bigger fish of the tank. Other fish: 2 Cardinals (both male), Blue Damselfish with yellow tail, Flame Angel, White Tail Bristletooth, a mated pair of Clarkies and the mandarin. Thanks for your comments!