betta biting his own tail???

well, got more fin issues . my boy is still losing chunks out of his tail fin. this is what i got goin on: 2.5 gal mini bow cycled about 4 months now(amm 0, ni 0, na 40-80) API freshwater test kit, i plugged filter intake with filter floss to remove risk of fins being sucked into intake pipe, filter is running VERY slowly so hardly any current to speak of, and replaced plastic plants with live java ferns & anubias. there is a small cave and a resin ornament, but they were both added after several of these "injuries" had already occurred. the only thing i can figure is that he is biting these chunks out of his own tail. anyone else have this issue? will he ever stop this behavior?

here is a photo of his current setup, in case it helps

betta house by big_cutie_monster, on Flickr

also, i forgot to add that i have never personally witnessed him doing this. i have read online about other ppls bettas doing it and that no one really knows why. maybe its because my 19 month old is loud and boisterous, not to mention an occasional tantrum-thrower. thats enough to stress anybody out... lol. otherwise he seems totally normal; eager to eat food, builds bubble nests and fiercly defends them, swims around, flares at reflection. i just dont know what more to do. its really frustrating