Whats Going On With My Fish.

[fishmaster2234]

I purchased a juvenile Black Ghost Knife, 2 Agasizzi Apistogramma, and a Golden Wonder Killifish. Then this morning when I woke up I found the BGK one of the Apisto and previously owned for eight months two gold barbs all dead. I couldn't figure out what happened, the pH was normal they were not overfed and everthing seemed to be normal. Then, about five minutes ago I saw my large silver dollar with something in its mouth, at first I thought it was a leaf but when I got closer it turned out to be my other apisto in its mouth !! I have never seen my silver dollar show the least bit of aggression to any of my fish. I thought that silver dollars were trictly herbivores can someone please explain and possibly tell me how to prevent this. Thanks in advance.

 

[fishmaster2234]

By the way when i found them dead they all had white layers around them I have never had this before in my aquarium.