Rosy Barbs are generally peaceful fish. They do best when they are placed with a minimum of 5 to 6 fish together.
Some individuals have been reported to attack other fish and nip the fins. This is mostly the exception rather than the rule.
One common reason for aggression is the tank size. Ensure that your tank is not overstocked. Rosy Barbs need their own space.
Rosy Barbs get stressed easily. Provide plenty of hiding spaces for the fish.