Fish have been dubbed to be the most aggressive things on this planet next to people, so it's pretty common that you'll find one fish chasing another fish. The thing is, is it chasing it to the point where it's shredding its fins, it's kicking up at the corner, because this will surely result in the fish probably dying.
If one fish is chasing the other fish, and the tank is big enough, the fish are typically compatible, one thing you can do is to let them re-establish their territories by changing the tank decorations. Typically when you add a new fish to a tank the existing fish that are in there are going to be very territorial and not want to see anything new in the tank.
So, for that reason, I recommend changing the tank decorations. Not physically changing them, but just moving them around, creating new caves, new hiding places, whenever you add a new fish to the tank. That will disorient the existing fish and make them re-establish new territories including or incorporating the new fish as part of its new environment so it's less prone to chase the new fish.
If you try this and it's still not working you could try pulling out the aggressor leaving them in a bucket with an aerator and a heater for six to eight hours. That should disorient them so when you put them in the tank sometimes they just happen to click in and get along with their existing tank mate. Or if you have the luxury of setting up another aquarium you can move the aggressor out into the other aquarium for a few days to a week, let the new fish get acclimated, let it start eating, let it develop territories, and then put the old fish in.
One way to prevent fish from fighting with each other is to purchase fish with this in mind. So remember, fish are aggressive, so they're going to pick on things that are the same size or smaller. If the new fish is slightly larger then it has a much better chance defending itself. Very rarely will you put a new fish in the tank and that new fish starts chasing all the existing fish. It's usually the other way around. So I recommend buying fish that are slightly larger than the fish that you already have so they can hold their own in the tank.
You could also try introducing the fish at night or shutting the lights off in the tank, shutting off the lights in the room, and introducing the fish when everyone's asleep. So they wake up the next day and all of the sudden there's a new fish in the tank. That'll disorient them and sometimes allow the new tank mate to be part of the new day, the new environment.
All these things can be done with success.