As stated earlier, a list of your current aquarium stock would be of great assistance in helping us help you find the culprit.
Male Guppies Male Bettas, and other longfin fish are more prone to developing finrot without any nipping from other fish, due both to the long fins being more likely to snag on rough decorations, and the fact that long finned fish have poorer blood circulation at the ends of their fins than fish with the original, wild type versions.
Sometimes this can result in spontaneous finrot from poor water quality, with an injury not even being necessary to start it.