Looks like the result of osmotic shock. Its a bacterial infection that took over due to aclimation from a very low salinity like most LFS keep their fish at to your salinity. Then a partial and too fast aclimation and the fins start to rot off as well as the gills have difficulty and burn. Its all the way into the tail base as I can see from the picture. And from the picture it looks like all fins are affected at their outer margins as is the mucous on the entire fish. It takes a very long time to recover from that. The scratching on rocks comes from the fact that its annoying and he is trying to knock it off. Ick is a secondary parasite as is the bacterial infection. The other things that can cause a persistent fin loss like that is a fish picking on him and taking chunks out of his fins, real poor water (as in tap water for make up water), poor maint. overall (lots of bacterial problems with nitrogen), and excessive stray current from a faulty pump seal. If you have a UV sterilizer running all the time, check for residual ozone. That will also burn the mucous and fin outer margins.