ummm, ideas on what this could be? im not a biology major... its been a serious consideration but at the moment im not... so unprofessional guesses, and im just listing whatever comes into my head here, so.....
catfish venom like you mentioned,
genetic defect - though if it is a genetic problem it might still be curable in the sense that maybe there is a vitamin deficiency that is causing it... maybe even one specific vitamin. maybe you could try and get foods that literally are its natural diet to make up for any lacking that processed foods have.
perhaps it has certain natural swimming behaviors that it cant exhibit in the aquarium - could your tank be too small?
maybe its a bacterial or viral infection.
cancer
did you say it was pregnant? maybe it has the remains of a poly fetus that failed to eject - i know thats morbid but im throwing everything i can think of out here. just trying to help, hope you understand. im trying to think like an scientist so sorry if thats too gross.
maybe its constipated
autism happens in humans, could forms of it occur in animals too?
missing a chromosome?
are there any pesticides or chemicals near the tank, even vapors like from an air freshener can. i always worry when we spray a deodorizer in the basement if it will hurt my fish. it is also a reasonable thought to mention that such a thing could only affect this one fish as it may be that the nature of the chemical can effect a certain enzyme that only this species makes, and the manipulation of even one enzyme can make a huge difference (i have taken 1 biology class so far lol)
maybe it got bit and things moved around in there and hurts to not be in anything but that awkward position.
maybe its an alien in a poly outfit.... and thats a sign iv run out of ideas, for now. does that help any?
ok well im off to art class, hope that helps some.