does your water temp flucuate? for sps's even a 2 degrees or more can spell death. Did you acclimate it via slow drip? Are the other corals sps or lps? The softies are much more forgiving to flucuations in water temps. MAke sure you have accurate thermometers in your tank and at least two of them. I set one high and one low. Also you may want to place one in your resevoir to keep an eye on that prioir to doing a water change, believe it or not if you have a powerhead in there circulating the water it can heat that water.