Why are my guppies fighting?
Like most livebearers, the life of a guppy is pretty much all about sex and violence. The reason why males are naturally colourful is to impress females and to help intimidate rivals. Like many social species, keeping small numbers of these fish actually amplifies the tensions that would normally be spread through larger groups.
As well as keeping a group large enough to spread aggression, outnumbering males with females and providing plenty of hiding places for weaker fish in the form of plants will help.