Are the fish the same species?? (do they look the same). The fish that has been there for a month has established territories. From your discription the clowns that you have are probably amphiprion ocellaris
(commonly known as ocellaris clowns
, false perculia clowns or *nemo* clowns). If they are the same species the fighting should
die down after a little while (maybe 2-3 days) and the larger clown will always be the dominant individual. The reason for this is that all clown fish are born males, they will form pairs and the most dominant one of the two will become female and begin to grow larger. The male will simply stop growing. If they are of different species it would probably be best to return the one you just got to avoid a war that will ultimatley end wih the larger one killing the smaller one.
Leave them be for a little while and see how they go.