I got a Pseudotropheus demasoni on Saturday. He was doing great until an hour ago. I found him at the bottom of the tank. My water conditions are perfect. I haven't seen any of the other fish attacking him. It's unusual to me because in the past when I've lost a fish there are signs before like they are struggling to breathe, or have missing scales and tattered fins. I've never seen one just drop dead like this. What happened?