Okay, in a matter of hours the whole bottom part of his tail is in tatters like the rip in the picture. I don't know what's causing it. I've done the pantyhose test on all of the tank decor. The filter current is baffled and very gentle, and he doesn't go near the filter anyway.
He is extremely active as usual, and shows no other signs of sickness. The water parameters are fine and it doesn't look like fin rot at all. Water is at a stable 80 degrees. Is it possible his tail could rip from too much swimming? He swims constantly.
I want to make sure they don't rip anymore.