Whats wrong with my black moor goldfish?

A picture would help, but I think it could be finrot or some kind of fungal infection. Are the patches fluffy at all? or does the fin look ragged/torn or eaten away? If so, then you should treat it with anti fungus and finrot. Adding the salt may have suddenly changed some of the properties of the water and that could have stressed the fish, leading to it being weakened and more susceptible to infection. Ideally if you were going to use aquarium salt, you should have put it in there to start with to avoid sudden changes in the water. However, now that the salt is in, it sould be beneficial and will reduce the risk of the fish becoming infected again, so you can keep adding a little when you change the water. Try and keep the concentration of aquarium salt in the water constant.