Generally when i fish stops feeding the first thing to check are the water parameters,in 90% of cases this will be the problem,get the water right and the fish feeds...simple as! it could be a simple thing like a drop in temp,look for the simple things first.
Regarding salt to use as a tonic i have had unbelievable results both with rays and other fish,however i also have to agree that if the water checks out spot on then there is no need for salt or other chemicals to aid the healing process of scratches or wounds,they will heal on their own,fish have amazing healing capabilities,infections are caused by poor water conditions.
Good husbandry is the key!
Good luck!