Hi,
I'm glad I just found this as I have had the exact same problem. A couple weeks ago I have a mystery death of one of my healthy female kribs, I was really shocked to find her dead and there was not really any explanation I did a 50% water change and just carried on as usual, since then no other deaths except……
I noticed that my red tail shark would hassle every fish including a 20cm pleco and a red finned shark (they were meant to be friends) then I came home last Friday and there was the red tailed shark DEAD he was sitting head up in the water vertically with his tail touching the stones and missing a few scales??? I figure that the pleco had enough of him as there isn't really any fish in his tank to beat him up????? :-? :-? :-? :-?