I am not sure what happened, but my female, Lilo has what looks like a gash on her back. It wasn't there when I left for work at noon, but my daughter noticed it when she came home from school. Lilo is in a divided tank with another female. The divider has slats and although I find it unlikely if Lilo were too close and Poly turned the right way it could be a bite. I don't know what she could have caught herself on. Maybe her little cave, but I didn't think it is that rough.
Either way will it heal on its own? Is there anything I can add to the water to prevent any infection? I will try to post a picture, but my camera isn't very good with close ups.