I would guess that it depends how close it came to any organs. I don't know much about the anatomy of a loach, but I mean that may be close enough to the eye and the brain to unfortunately have to say goodbye. I was under the impression fish only have nerves in organs (could be way wrong on this), a pain response is a bit different than being infected by a toxin, but then again swelling near the gills may complicate matters as well. My old clown knife got stung in the throat and other than having a permanent red bump he was just fine, even day of the sting you couldn't tell any sign of distress. Stranger things have happened, crossing my fingers for you.