We have had our Kole Tang now for about 7 months. He has always been an active eater and quite active. He was eating fine last night but this afternoon I noticed he was hiding out in the rockwork and did not come out to eat when i fed the tank. The parameters of the tank are 0 Ammonia, 0 Nitrites, 5ppm Nitrates. There have been no new additions to the tank for several months. Any thoughts or ideas?