If he is flaring a lot of the time, a betta can blow his tail. The membrane is only a few cells thick, longer fins blow more easily due to rapid movement in the water. My male did this when I first got him. He would be flaring constantly at his reflection and his tail looked like a shredded flag. You just have to keep his water clean until he calms down enough to let it heal.
My roommate says I'll die by giant turtle.