How did you acclimate the colt? Was is under MH
lighting at the store? If not, you need to acclimate it to the lighting as well as the water conditions. Light shock can kill corals as fast as water conditions. My guess is the change in environment stressed it enough that it couldn't fight off infection. If it came from under VHO
lighting, you'll need to start it off in a shaded area in your tank. Give it some time to get used to the new water (after careful initial acclimation) and slowly move it into brighter light. You may find that it's happier in the lower areas of the tank where the light isn't so bright.