Options for treating hard water spots from a dishwasher include adding white vinegar to the automatic rinse aid cycle, reducing the amount of detergent you use and installing a water softener. The spots form as the heating element from the dishwasher warms the interior to evaporate the water.
If the commercial rinse aid you normally use in the dishwasher does not prevent the spots from forming, try filling the container with white vinegar. It is inexpensive and helps to dissolve the calcium and other minerals that form the spots so they rinse away in the drain. Vinegar is also a natural odor eliminator, so the dishes come out of the machine smelling fresh.
With a dishwasher, more detergent is not always better. Some users add more detergent to eliminate the hard water spots, but too much detergent can leave a soapy film on the glass. Measure the amount of detergent for each load carefully.
If your efforts prove unsuccessful, the problem could be with the water supply in the home. The harder the water in the house, the more spots it forms on the dishes. Installing a water softener helps to reduce the spots. The softener eliminates calcium and magnesium from the water supply, making cleaning easier and improving the taste of the water.