The only way to prevent hard water is to remove minerals from the water.
The Water Softener Cartridge does just that when inserted into the Clear Inline Hose Filter Canister. The cartridge works automatically to remove calcium and magnesium from the water, thereby improving the texture and quality of the water that touches your vehicle.
A water softener works by replacing the calcium and magnesium ions with sodium ions. Sodium ions are "soft". They don't produce scale and they don't stain. They change the actual texture of the water, making it feel "slippery". This causes water to sheet off the vehicle more effectively. Water that remains on the vehicle will not leave spots.
The amount of sodium added to the water through the softening process is minimal. In fact, there's more sodium in a gallon of milk than in a gallon of softened water.