Grains per gallon is how your water hardness is calculated. You will see the abbreviation for this (GPG) in relation to the most hardness elimination amount for every water softener. The upper the GPG, the higher the grain potential you will need. Use the correct salt from the water softener. https://water-heater-installation54219.alltdesign.com/top-guidelines-of-tankless-water-heaters-49649229