It doesn't look like fin rot from the photos, of course they have a lot to be desired. I'd assume one of your more aggressive fish in the tank is nipping the tail. I can't tell you what you should do, but if you have another tank, I'd suggest removing the possibly injured fish, just be certain. I'd also check the Ammonia and Nitrite levels, with a quality test liquid test kit, (API, Salifert).
edit:
both ammonia and nitrite can poison fish, and cause their fins to 'rot', their skin to flake away etc..