I had not heard of this fish, so I googled it and found 3 articles that all state it is a particularly aggressive tetra known for fin nippage. (Yup I said it) apparently they are ok in a tank w/semi-aggressive species, can be kept as a single specimen but is better with 3-6 in a school. They also suggest dim lighting, hiding places like caves, and they apparently like to jump when stressed! I would think it was entirely possible it nipped the end of a tail off. Good luck with your dilema, it looks like a really cool fish.