Many fish should never be exposed to salt even for medicinal purposes, this would include Appistogramma Cichlids, wild caught Corydoras, most Pimeloid catfishes and most South American Characins (tetras, pencil fish, silver dollars etc) as they have evolved in areas which have no measurable salt content to the water at all. While the harmfull effects of salt may not be seen immiedietly the damage is being done in secret, as the fish has not developed a way to excrete salts from the body the salts accumilate in the kidneys until eventually over time the kidney hardens and the fish dies from kidney failure. Please be sure to only use salt in tanks containing fish which are proven to be salt tolerent.