Let himself get sucked into the filter intake?
That definitely should not be happening, lol. What kind of filter do you have in the tank? If it's a Tetra Whisper of some kind, you can usually put a strip of pantyhose over the intake slots, or maybe a thin sponge. If it's an external with a descending intake pipe you can usually put a sponge around them, or again, a piece of pantyhose.
Is there a chance it could be tail biting instead of fin rot? Your water sounds so clean, and with aquarium salt and Indian almond Leave (which is supposed to help treat fin rot, right?) present I can't imagine fin rot being an issue. Not that it couldn't, somehow, anyway. But, maybe there's just a chance he's stressed? Does he have a lot of plants or decor to hide in? Regardless, I'd suggest fixing tampering with your filter to prevent anymore accidental suctions and see if he doesn't improve.