Just a thought, could it be the filter intake tearing his fins?
Sometimes their fins can get caught in those intakes and tear. You can get a round sponge that slides over it. I wouldn’t turn it off, he needs the oxygen.Thanks for all your help Russel! A log would be cool, I’ll see if I can find or make one.
I guess that could be possible Sara? The filter is smooth rounded plastic so I don’t think he is snagging his fins on it, but the current it makes seems to be a little too strong for him, he tends to avoid swimming out where it’s expelling the water. I guess I can try turning it off since the cartridge isn’t even in it rn.