Have you ever heard of such a thing...a truly psychotically aggressive *guppy*, of all things. I got some new male guppies that were unusually aggressive, yes, aggressive. And upon close observation, it appeared to be one in particular, but I was late in discovering this and one poor guppy was murdered by him. (And there are no females present.) I named this homicidal maniac Charles Manson. I've kept guppies for quite a while and it is really rare that there is any aggression, much less homicide, between them.
Anyway, I moved Charles Manson to a tank with no guppies...the tank contained 9 female bettas and 2 dwarf gourami. I didn't give even the slightest thought to any possibility of Charles Manson committing inter-species murder. Well, in less than 24 hours Charles Manson killed *five* of my nine female bettas before I could stop him. But when I did discover his mass murder spree, I euthanized him.
Charles Manson is dead now, but not before he killed six. Sheesh.