I wouldnt exactly call it 'play fighting'. Their behavior can be sourced to a couple of reasons and without knowing more information, its hard to say which it is. Aggressive behavior in goldfish is primarily caused by inadequate tank size/overstocking or breeding behaviors. Breeding behaviors can be seen in female only and male only tanks- there doesnt have to be members of the opposite sex present, just a dominant fish. As long as they have plenty of room and no one is being hurt/overly stressed, I wouldnt be too concerned but I would just keep an eye on everyone. Goldfish wont fin nip each other but they will push and bump and chase to the point of injury or exhaustion. Please ask if you have any questions!