If you don't have a hospital tank, the best you can do is not stress it more than needed. Feed it well and maybe make the night cycle a little longer as rest helps them.
If you do have a hospital system, you can put it there after a fresh water dip and treat it with ant-fungal and antibacterial. API Melafix marine is a good alternative. Look into prazipro as an alternative for in tank treatment although might not address exactly whatever your fish has. The tail appers to be some parasite related condition, so you may want to look into API's General Cure.
Finally, if you have a tankmate that's injuring him (which I doubt), address that issue as well.