Mine try to nibble at me when doing water changes but nothing bad. I had one of them jump out when going for food. I try to be very careful when the lid is open now. They seem to be healthy but look pitiful with their curvy bodies.
There are a lot of things that can affect development, however I can’t speak on the exacts... parameters, lighting, disease without being specific lol.... hopefully one of the others I have tagged can chime in with more info.
I've seen killies deformed like that, but they were always deformed from birth. Not sure what could cause it in partially grown fish. Only thing that comes to my mind is water chemistry or lack of proper nutrition.
If the curvature developed with time, with that species, I would euthanize the fish asap. For some reason, that species, lineatus, seems very prone to Mycobacter marinum - fish tuberculosis. It is an incurable, communicable bacterial disease. The fish will develop cysts to encase the bacteria, and if the internal lesions are along the spine - that's what you see. Teh cyst grows and the spine can't develop
If you dare to mention fish tb on forums, people can get upset. We don't all have labs handy to wait a few months for a culture to come back, to be certain. I will happily admit I could be wrong - but the species, the fact it is a fishfarm colour variety (not wild), and the development of the deformity in a previously healthy fish add up, to me. Tb is dangerous when the cysts finally break down and the bacteria encased escapes to spread. I would not want that fish in my tank when that happens, as it inevitably will if the poor thing is afflicted.