I say let it live as long as it can! Just because odds aren't in its favor doesn't mean it shouldn't have an equal chance at life
On top of what Ive said what happens if it reaches adulthood and somehow breeds and passes along those genes causing countless more deformed fry. Either way the result is bad. Also have you ever seen one of those documentaries on human conjoined twins? they are constantly in pain (at least the one that I saw) and their bones/internal organs don't function right leading to a stressful and painful life.
Uhmm, that's how genetics work and how such twins are formed.
Maybe you should educate yourself a little on the topic.
I have to jump on the band wagon here....fish lack the necessary pain receptors in their brains to experience pain the way humans and other animals do. While fish have nociceptors — sensory receptors that respond to physically damaging objects and events by sending warning signals to the brain — these receptors don't function in the same way in fish as they do in humans.
You have to remember this is a lower life form, their brains are set up more for instinct. Of course they will react if you pull a fin off but not in the same way as if you pulled a leg off a cat. I am not saying they will not feel it, it just just not the same signal we would receive.
I have a masters degree in genetics and have worked a few years in ichthyology. My specialty is cell signaling...aka..they ways cells communicate to each other. Sorry but the physiology isn't there. But thanks for asking.