First off, table salt is perfectly fine to use in a tank, in the right amounts, for the right reasons. Not eating is not one of them I am aware of.
Water quality should be measured to determine if it is an issue. If you have very high nitrates, and you do a huge water change, that can actually stress your fish as well.
It doesn't matter if someone paid you to take the fish, or how much it cost. It is in your care and you should do your best to keep it healthy. If I were you, I would do a little bit of reading on this species and determine what is the best diet for it. Then I would go out and buy me a test kit so I could monitor the water quality and know what is going on in the tank. (The cycling article linked below is worth a read. While your tank is (hopefully) cycled, it will give you an idea of how fish waste is processed.)