I had 3 large colony of Zoa when I first setup my tank. They were doing very well, growing like crazy. After I add SPS and upgraded to a larger sump and maintain a cleaner water quality, all the zoa slowly melted away. I moved the remaining to the sump and they seem to do better. (there are a few dozen of polyps that attached to the large rock in the DT, they seem to do fine (very slow growth though)). I guessed that they need to have some dirty water to thrive!