I came home last night, and one of my female swordtails appears to have suffered some rather serious fin nippng. Most of te wavy white part of her rear fin is simply gone, and she's just sitting on the bottom of the tank. She's still capable of swimming, but isn't trying to get food or anything. Is there anything I can do for this fish to aid in her recovery? If not, is there a humane way to put her down? Second part of the question, I believe it was my male swordtail who did the fin nipping. I've had him in a 20g tank, with 2 female swordtails, 2 female guppys and a male guppy for about 2 weeks now. He's never shown the least tendency towards aggression or fin nipping, but I watched him do some fairly aggressive chasing last night, and of course came home to a swordtail, without much of a tail. Is this typical behavior? Do I maybe just have an overly aggressive fish? Yesterday was also the first day I had ammonia readings from the tank, but it was still well within safe ranges, and I did a 20% water change anyways. Could the ammonia be annoying him and making him aggressive?