You need to buy some patients now and do not add anything else to your system until you resolve all of your water parameters and are able to keep what you have alive. sometimes the corals will recover from bleaching, you will just have to wait. check out the helpful articles and you will find a article or two on coral acclimation. bleaching can be caused by environment stress, water parameters, lighting, etc. what type of coral are we talking about here. also is it correct to assume your system is but 6 months old? system is not yet matured enough to handle much of a bio load so read the articles and you will find a lot of information to help you with your project. long term success is based on the hobbist understanding the principles of the difficulty that is at hand. if you do not you need to read or get the information you need to understand how to deal with or prevent the problem. just doing something because others do it is a recipe for long term difficulties. good luck and keep reading the helpful articles. all your questions will be addressed there at some level. temperature can cause bleaching also. so if you take the coral out of the bag and plop it in your system with too great of a temperature difference you could experiance coral bleaching. also you want to make sure no air has been traped under the coral. good luck with your project.