Hard water, with a total level of calcium, magnesium, and other minerals at 7 grains per gallon or 121 milligrams per liter or higher, can diminish how well your dishwasher cleans. If your dishwasher's heating element is covered by a whitish crust or glasses emerge cloudy or spotted, you might have hard water.
To determine the hardness of your water, contact your water company. If you don't have municipal water, use a home test kit, which you can pick up for $10 to $25 at a home center or hardware store. If you have hard water, you may need to increase the amount of detergent or consider installing a water softener. (Some Miele dishwashers have a built-in water softener, to which you add dishwasher salt, available online.) And to help your dishwasher work at its best:
Refer to your owner's manual for details. The harder your water, the more detergent a load needs. GE suggests that you calibrate a model with its SmartDispense feature according to water hardness so that the proper level of cleaner is dispensed.
To prevent the common hard-water problem of spotting and to help dishes dry better, most makers recommend that you add a rinse aid. Again, check the owner's manual.
If you see white residue inside your dishwasher, you can occasionally try to dissolve it with distilled white vinegar. Instead of using detergent, place a container with 2 cups of vinegar in the bottom rack and run a normal cycle but without drying.