I have seen some of my younger barbs attempt to be agressive. I don't know how old they were when I got them, but i would guess they were 1" from nose to the tip of their tail. It was slightly comical to watch, because the younger one thought he was "chasing" an older one, but in reality, the older one was busy chasing someone else, and wasn't threatened by the older one. I also have one tiger barb in my hospital tank because he has an infected sore right above his mouth, which was likely caused by a small fight.