Great point pink diamond about the salt resistant parasites and bacteria.
I think everyone should know this though, it is super important in terms of fish physiology. Salt also affects the osmoregulation of the strictly fresh water fish as well, it disrupts their ion water balance which is not that good in the long run. Freshwater fish have a higher concentration of salt in their bodies than their surroundings, so their bodies are built to manage their ions in their body based on their environment. The salt will move from a high concentration to a low concentration as will water. The fish will always be focusing on keeping their ion levels constant because it will be moving from them into their environment.
When switching the environment to a higher salt concentration the fish will get more ions, yes this is true but not really that good...because,they will lose water to their environment in the process. This is because now the environment has a higher ion concentration than the fish. As the ions move into the body, water in their bodies are pulled out....fresh water fish are not equipped to have to compensate for water loss in this way as they do not drink.
Salt water fish do drink to gain water! And they also have specialized chloride cells to get rid of excess ions, which fresh water fish do not. That is how salt water fish manage their water concentration when the ion conc in water is higher than that in their bodies....thus you are inadvertantly dehydrating your fresh water fish by adding salt in the water...basically.
I would always go with salt baths
over putting salt directly into tanks. Every once and awhile straight in the tank should be okay, but you should think about the internal effects on the body it is having. The fish has to work harder to maintain homeostasis. That's my honest opinion and my reasoning for this. It is a little more complicated than what I described but I don't want to bore anyone with my physiology nerding out
You don;t have to follow my advise or anything I just wanted to explain why I always tell people SALT BATHS over right in the tank.