There is something wrong with my goldfish.. what should I do?

I recently bought three goldfish. I put them in a 20 litre aquarium (the tetra brand). One of them, is chasing the other around the tank, and this has been going on for about 30-45 minutes, with occasional stops. The show no sighn of stopping completely yet. I have read somewhere that they do this before they start breeding. Is it just a coincidence, and if it is not, what should I do?