okay the above post might help me out. How does this sound:
one of my bettas had fin rot. I don't have another tank and figured that if it was going to spread to the other, it probably would have so I started treating it but didn't separate them. I don't know if this was a mistake or a little or big one, if it is. I didn't mean any harm by it, obviously. I just thought that it would be fine.
we started doing 80-90% water changes and treating with aquarium salts after every change. I also changed the filter pad that I was using to baffle the filter with and changed sides that it was on because the water changes were bringing up scum on the other fish's side. I guess that the stream from the filter and the difference of sides may have thrown the other fish off and caused him to bite his tail. Does that make sense?
I just think that if the other fish had fin rot, there's no way that it would have gotten as much of his tail as is damaged.