As mentioned, it is possible that the prolonged stay at the LFS in a small bag with no water change or feeding pretty much contributed to the ill health of the fish. Do not rely on salt to cure every disease though. It is not necessary for some fishes, and can even be unhelpful, especially for scaleless fishes. Even armored fishes like Corydoras, do not like salt to be added to their tank.

Keeping the tank clean, maintaining the filter and doing regular water changes are key to keeping the tank inhabitants healthy. Checking the freshness of the food you are feeding, is also crucial.

At times it is best to just have a bare-bottom tank, with plants tied on driftwood just for aesthetic purposes. This allows you to keep the fish, and yet prevents issues related to the gravel bed.