It is said (and I belive it ) that in the wild ,,that fish like barbs,and tetras, school in the hundreds and nipping and bullying is how they determine their status or pecking order.When kept in smaller groups, the bully or bullies will often times pick on other smaller or weaker fish and wounds or torn fins can become infected which in turn ,,can result in finrot or bacterial infections. This is why it is important to keep the water quality as clean as possible through regular water changes,and not overfeeding or overstocking. With good water quality,it is much easier for fish to use their immune system to lessen the chance of infections,finrot etc from escalating. And sometimes,, despite our best efforts,fish die.
The most important medication in your fish medicine cabinet is.. Clean water.