Just had my first loss :/

A week and 3 days ago I purchased a dottyback and entered him into my tank. I had a cinnamon clown in there prior to him about a week. I moved things around in the tank to try and do away with some already established territories but I guess it didn't work. Last night I saw my clown in the dottys cave and got him out and turned the lights out to keep him away. (Was 6 hours into my 10 hour cycle). This morning I find the dotty on the rock lying motionless with a whiteish get film on him. I got him out of the tank and watched him for a few minutes and no motion. Man this sucks. I thought they were okay seeing how there was no problem for over a week. What happened? Why over a week later do they fight. The dotty was keeping his distance and I was even feeding on opposite sides of the tank to keep from fighting