You can usually Google the water hardness in your area—it’s measured in parts per million, milligrams per liter, or grains per gallon. A higher number means you’ll need more detergent (probably of a higher quality), plus rinse aid to offset the effects of the extra minerals in your water supply. If you get the dosage right, your dishes should come out sparkling clean (awesome on its own merits), and mineral scaling shouldn’t build up in the dishwasher as quickly, either. Follow the dosage guidelines in your dishwasher manual, and prepare to experiment a bit.