Help! I have two betta fish separated by a divider in a 2.5 gallon tank. The fish that is not on the side of the filter has what appears to be holes in his tail. There are sections of his tail missing and the tips are raggedy. I thought it was one fish nipping at the other through the divider, so I separated them father with some thin plants. I've been observing them and they don't seem to go near each other anymore, but his fin is getting worse. Could he have fin rot? If I put salt in the tank for the fin rot will it harm the other fish (and the black mystery snail, which is on the side with the filter and the unharmed betta)? What's wrong with my fish? Help!!
Two pictures of the situation: