Join Date: Jan 2013
Location: Um, I don't like Stalkers.
Looks like a case of a tail biter to me... Your fish is heavily finned so there you go. It might also be flaring, but biters are common. Silk plants are the ones that don't tear fins. You can do a pantyhose test. Rub pantyhose on the decoration and if it rips, it can do so to your betta's fins.
This will go away on its own, warm and clean water is the best preventative and treatment. Besides, he can bite while in treatment so it might all be for nothing.