Not specific to goldfish, but a University of NM aquaponics publication says,
“Water Hardness
Water hardness is a measure of positively charged ions, particularly calcium (Ca2+) and magnesium (Mg2+). To- tal hardness is the sum of the concentration of Ca2+ and Mg2+, as expressed in ppm calcium carbonate (CaCO3). Hardness can range from soft (0–75 ppm) to very hard (>300 ppm). Dissolved calcium in the water aids in os- moregulation and relieves stress in fish. Levels should be maintained between 50 and 100 ppm.”