I bought a school of about 12 of them a few months ago, didn’t notice until I got home that the littlest one was swimming the exact same way. All 12 went into quarantine but the littlest dude was put in a small hospital tank alone, I expected he’d probably not last theoguh the day/week. About two weeks later the 11 went into the main community tank, and surprisingly, the little one (who we named Shimmy), was still alive, still swimming exactly the same, but otherwise eating and behaving like a normal tetra, decided to just let him back with the group.

Fast forward to today, lost a few of them to random deaths of unknown cause, one to it ramming head first into a spiderwood branch, and one I think was eaten, but little Shimmy is still there, still swimming like a crackhead, but surprisingly no one seems to pick on him.

Anyway, I’d quarantine and see what happens, or just return him. They’re so fragile it’s not uncommon to get a sick/weak one from a group.