I have six fish im my brand new aquarium. The tank is 21 gallons, with plenty of plastic plants ( i plan to switch with live ones later). I have 3 (young)black lyretail mollies and 3 (young) red platies. The platies (two male, one female) seem to get along fine, with each other and with the mollies. However, although my research and the aquarium store assistant assured me that these fish were ideal for a community tank, the mollies are constantly fighting. Is this just territorial? I can't tell the sex of the fish so I don't know if two males are fighting over a female but it is unlikely, should I leave it alone or is there something I can do??