My betta fish has fin rot?

I did a full water change. I added 1 teaspoon of salt per gallon for fin rot. I have read somewhere that to prevent fin rot and other stuff, you add 1/2 teaspoon of salt and when curing it, you add 1 teaspoon per gallon. Is this right? Will I kill my fish doing this? This is NON Iodine salt by Kosher. The same as aquarium salt.