Sounds like tail biting to me. I'm pretty familiar with it because both of my males are fin biters (one much more than the other).
But that's what my major tail-biter's fins looked like when he first started doing it- not bites, but tattered and shredded. Then he would start biting it short.
The trigger was when his filter got gunked up and was coming out the overflow, making the water current all crazy. But he's in a different tank now, on the opposite side of the divider from the filter, but I think once he started biting it, he realized it made him feel lighter, so he just continued to do it whenever they started getting heavy again. He even keeps his ventrals short.
Anyway, your filter could be the trigger. If it makes it too difficult to swim, he might start biting his tail.
My other male bites his dorsal fin and sometimes a little bit of his tail.