Few things more depressing when everything seems perfectly fine than finding a dead fish first thing in the morning when the previous evening it was swimming around without a hint of any trouble. This morning my regal tang that I have had for over two years was lying flat on the gravel, no signs of any damage whatsoever, just dead. My yellow tang is also nowhere to be seen, still waiting for report from home to see if this has now appeared, but I am not hopeful at all. I have had this before with tangs, just suddenly die when all the other fish are fine. I won't be able to check my water until this evening, and then if there is a spike it will be Wednesday before I can do a water change. Of course you try to think of anything obviously different that I did yesterday. I introduced two new corals and used a putty that I have used before to secure them in place. This is the only thing I can think of that may have caused this.