Salt is something that should always be kept in a tank. Read this. You might learn something new
Fish have organs that enable them to adjust to small changes in the concentration of salt in the water. On the other hand, pathogens such as bacteria and protozoa are simpler life forms with no internal organs to regulate changes in salt concentration. Most common skin parasites live on the skin surface, not beneath it, so are more vulnerable to low salinities. The concentration of salt can be changed to weaken pathogens present without harming the fish, thus giving sick fish a better chance of recovering.
Low dosages are also believed to help clear the gills of excess mucus that builds up in cases of parasitic disease. Some fish hobbyists might use salt as an as a one-off treatment in an emergency, when the fish is suffering respiratory distress and no other alternatives are available. However, extra care needs to be exercised in such cases, particularly if the fish has already been weakened by disease.
When fish are stressed or diseased, their osmoregulatory systems do not function properly. When fish suffer “osmoregulatory dysfunction,” they find it more difficult to obtain sufficient salts from the water, and get rid of excess water. This means that they lose bodily salts to the water. Salt replaces the lost sodium and chloride ions that stressed or sick fish need. Adding salt to the water helps fish to reduce the amount of salt that their bodies are losingthis principle is akin to that of providing hospital patients with saline drips.