One way to cut down aggression in an aquarium is groups, you should never buy your fish in 1s.
All fish swim in groups, as for guppies if you just put males into a tank they will group up and fin nip other fish. with guppys you want a ratio of 1 male for every 3 females this will keep him happy but that wont stop your other fish nipping him.
By putting fish in groups that keeps them occupied with there own kind and they tend to leave other fish alone.
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