Reef salt is probably OK but expensive.
You can buy 40 pounds of rock or pelleted salt at Lowe's for about $10 (sold for home water softening units). I would recommend starting with 1 teaspoon per gallon. That level can remain in the tank without needing to remove it, unless you want. Max level if you need to actually get it higher is 3 teaspoons per gallon. When actually treating for ich or velvet you start with 1 teaspoon, then add a teaspoon every 24 hours until you get to 3 teaspoons per gallon. You also raise the temperature to 80 or maybe a bit higher. But since ich cycts can live through this high salt with out a problem and then hatch out, you need to maintain these conditions for about two weeks to be sure to get rid of ich.
However what you describe is normal. Fish itch just like other animals and scratch. I would not treat unless with only one teaspoon per gallon. Higher salt promotes a fish's slime coat which is their defense against stuff that might attack them.
One other thing. When you add salt, always dissolve it first. If you throw in a lot of salt crystals or pellets, they go to the bottom and dissolve. Around this solid is a very high concentration of salt dissolved in the water, high enough to kill fish, particularly catfish and some other fish if they do not get out of it immediately. If you have a HOB filter you can add the solid salt directly to it. Or use some tank water to dissolve the salt and add the solution to the tank.
