I brought home some platies from the LFS this weekend. After placing them in my qt tank I noticed one of them is missing a pelvic fin. Pretty much just has a knub on one side. She swims just fine but I’m wondering if it possibly a birth defect, injury, or a result of possible fin rot? Is there anything I should do to check on the fish? Is worst case scenario that it is fin rot, and if so what should I do next? Any help is greatly appreciated.