Leporinus affinis an odd fish which lived peacefully with small tetras and barbs ect but would attack on sight any new fish. Everytime a new fish was added the whole tank had to be changed around to confuse it about its territory. A lovely fish though.
The people who advised adding salt, did they say what the salt will do? It isn't magical. Salt has some anti bacterial, anti fungal and anti protozoa properties it will also help ease some osmotic problems and that's it. Because this condition has gone on for this length of time I think it is safe to say that it isn't anything to do with digestion, blockages ect because if it was the fish would be dead by now. This leaves a true swim bladder problem and that is extremely difficult to cure. In a minority of cases an infection is the cause but when it is internal the usual antibacterial remedies are generally ineffective but an injection given by a vet would be effective IF this is caused by an infection. If the cause is physiological then quite simply there is really nothing that can be done other than surgery!!! Add salt if you feel it will help but all you are likely to achieve is slightly lowering the O2 level.