Why are your fish and chasing each other?

If they are the same kind of fish and it's always the same fish that's doing the chasing, then your fish are different genders. The male fish is the aggressor.

It's usually not a problem, but if the male fish is way larger it can nip off the other fish's fins after a few months bit by bit.

If you notice that the chased fish's fins are a bit tattered or broken, separate the fish from each other or add another female fish.