This may be the most important article / video folks will see this Spring because for a LOT of people with sick fish, SALT at 0.3% may be the fastest and easiest way to limit fish losses AFTER testing the water to make sure it's not as simple as a low pH (ha ha ha)
If it's NOT water quality, maybe it is a parasite and MAYBE you can nail it down with Salt.
Here's how. (Using Salt to Control Parasites and More)