One tablespoon per FIVE GALLONS for general maintainance, don't add more when topping off the tank, do add more when doing water changes but only enough to replace the ammount of water you removed.
2 TEASPOONS per gallon for treating ick and some types of bacterial and fungal ulcers.
3 TABLESPOONS of salt per GALLON is suitable only for disinfection dips for feeders and fish you can't quarantine, they are added to a bucket containing that concentration for no more than 10-15 minutes and are removed immediately if they show signs of distress, it will kill many fresh water fish in under an hour.