Betta fish dying after fin nipping?

[PseIloveyou]

Fellow Betta owners, allow me to set your mind at ease by saying I have 6 Bettas in 5 gallon-10 gallon heated, filtered, naturally planted tanks.The water gets changed every two weeks (50%) since plants reduce the need for changes and 1 100% monthly.The water is at 80 degrees and the parameters are checked every few days. Stress coat is the only chemical in the water.When I fed all of them yesterday morning (2 pellets), they were all fine. In fact, all of the boys had built large bubble nests to show me. When I got home from work, I checked on them all. My twin tail halfmoon was going around in a circle nipping his fins. He bobbed around the tank in this continuous circle and at one point got turned upside down and continued it. I moved him to the hospital tank, adjusted the heat, and wrapped a towel around him--hoping he would calm down. I checked him after feeding the others again and he had died. I supposed it was from drowning since he had no interest in coming up for air. No color change, no loss of appetite, no other signs of this behavior. He had been in this tank for about 6 weeks. I'd had it about 2 months and I believe that he was around 4-6 months old--based on size and when I got him. His tank mate (he was in a divided ten gallon) is acting fine, but I probably will conduct a 100% change to be safe. This is such a weird occurance and not one that I have seen in 13 years of betta keeping. I've consulted the help of some friends on Bettafish.com, but I was wondering if anyone else could offer some light to this scenario....Such a strange one.