I previously had him in a 5.5 gallon with a few guppy fry, and he was fine until last week when I came home and noticed that his tail fin had been totally shredded since that morning! There were no sharp objects or plastic plants in the tank (I've had problems with split fins before when there were plastic plants in the tank, so I don't use them anymore). And I'm sure the little fry couldn't have chewed off his tail all at once like that. I've ruled out fin rot since there were no signs of red on the tips, and his fins looked shredded rather than disintegrated. So, did he do it to himself? Does this happen with Bettas sometimes?
I've since moved him to a 10 gallon, which has a better filter and heater, and I've been using Melafix to try and heal him up. But I don't understand what happened to him! Any idea? He also happens to be constipated at the moment, which has never happened before either. It's strange, my other Betta NEVER has any problems in comparison.