The way I like to do salt treatment:
I place the fish in a smaller QT container and I pre-mix the salt 1tsp/gal in a clean 1gal jug, this is to make dosage and 100% daily water changes easier, I treat for at least 10 days.
I will make a 50-75% water change on their tank before I put them back in it after salt treatment has ended in 10 days.
If you treat in their tank, add 1tsp/gal of salt and make 50% daily water changes and add back half the salt, however, treating in QT is safer and easier with less risk of overdosing the salt.
Treating in QT also give you a place (their tank) to put the fish fast if they have a bad reaction to the salt, its rare but it can happen.....