Some seem to have a dominant personality, and others don't. They DO seem to copy each other, as Chels said. What one does, the others follow suite! And sometimes a group of guppies will all pick on one particular guppy. I have no idea why. Also, male guppies are always thinking about breeding, so will "bug" other males if no females are available. I've stopped keeping guppies because of their health issues (modern breeding for profit) and because of the "group beating up" thing. I was ending up playing musical guppies, constantly having to remove the more outgoing, aggressive ones and putting them into another tank so they wouldn't bug each other literally to death.
