They will definately do that. I just lost a hippo tang I had for five years. I had to move the tanks, and the next day he was gone... I am guessing it was because I had to take out all the rock and some of the sand from the DSB got stirred around. It was probably killed by bacteria. Oh well, everything is back on track now. It doesn't matter how good you are everyone losses a fish from time to time. It is just too bad you lose the fish you want and not the ones that are mean.