Have you checked the water quality?
Often fish get aggressive if there is a problem with water quality. Which can also cause a raggedy tail. (But also getting bit by another fish.) One might look like a chomp mark, and the other, more frayed.
If you don't have a test kit, do a couple of 25% pwc
one right after the other and then after a few hours or the next morning do a 30-40%.
That is to make the changes in the water quality more gradual, the difference of fresh tap and older tap water.
Make sure the temp is the same and the water is dechlorinated.