Why are my fish fin-nipping each other?

I got 2 goldfish yesterday that were in the same tank at the pet shop. They were fine at first and actually got on well, but now they seem to chase each other around the tank (sometimes) and nip each others fins. Why are they doing this? I don't know whether they're fighting or not because they do seem like they're just playing, I'm just not too sure. What can I do other than separate them? Thanks for your time.