Yes, food salt. But read the ingredients because most food salt contains anti caking agents and some contains iodate. If yours does not have these, it is safe to use, but if it does have them don't use it.Fish shops usually sell 'aquarium salt' which is the same but without the extra ingredients.
Try lots of extra water changes first as the damage could heal naturally. If it doesn't work, use salt at the rate of 1 tablespoonful for every 5 gallons (or 1 x 15 ml spoonful for every 20 litres). Dissolve the salt in some tank water in a separate tube, then when it is all dissolved pour it into the tank.