How much detergent should I use based on my water hardness?
With soft water (0–3 grains hardness), use a minimum of 1 tablespoon in each cup for POTSCRUBBER and NORMAL WASH cycles. With medium water (3–7 grains), fill both cups half full, and with hard water (7–12 grains), fill both cups completely. Note that water above 12 grains is extremely hard and a water softener is recommended. (Page 7)
From the official GE GSD820 Dishwasher manual, page 7
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