I did a water change in my 110g last night. When I came back into the room (approx 15 min later) my emperor cichlid was upside down at the bottom of the tank, dead. He has been in this tank for about 4 months now. He was 7'' compared to 3'' when I put him in. I did a 20% change the same way I have always done it. The temp of the water was fine (80` outside). All of the other fish in the tank are perfectly fine. Does anyone know what happened to him so I can try to prevent this from re-occuring? Thanks for the help.