What could have happened to my Betta's tail fin?

[CDederick]

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.

[Adastra]

Many bettas bite their tails as a reaction to boredom, stress, and other factors. Personally, I would forgo the use of melafix. It contains the ingredient melaleuca which some studies have shown to be harmful to the labyrinth organ of the betta. The fins will heal perfectly fine with clean water. You could use aquarium salt/kosher salt for a few days to encourage healing.

After that, you need to get to the root of the problem. Carefully examine your fish's living conditions for possible environmental stressors such as poor water quality, temperature fluctuations, excessive current, lack of plants and hiding places, etc. Some fish bite themselves for reasons you may not expect. I have had a tail biter who bit himself when he was in complete darkness--once I added a nightlight, he stopped. I had another that bit himself because he didn't like a certain decoration--once it was removed and there was more room in the tank for him to swim in, he stopped.

[CDederick]

Thanks, that's good to know about Melafix and Bettas. I'll just use salt then. His fins have begun healing ever so slightly, though I don't assume they'll go back to being the nice rounded shape they used to be. He must have bitten his fins because he was uncomfortable with his tank, but since he's in a bigger one now the problem should stop. Strange to imagine him being able to reach his back fin...