20 Chapter 1 Introduction to Statistical Quality Control, 5th Edition by Douglas C. Montgomery. 39Copyright (c) 2005 John Wiley & Sons, Inc.Chapter 1 Introduction to Statistical Quality Control, 5th Edition by Douglas C. Montgomery. 40Copyright (c) 2005 John Wiley & Sons, Inc.
Water hardness is a measure of the dissolved mineral content of water. There are two types: the general hardness (GH) and carbonate hardness (KH). When keeping freshwater fish and plants, the GH and the KH of aquarium water should match their natural habitat.
The hardness of water is measured in grains per gallon (gpg). One grain of hardness is equal to 1 / 7,000 pound of rock. Your municipal water supply may measure water hardness in milligrams (mg), liters (L) or parts per million (ppm). One gpg is equal to 17.1 mg / L or 17.1 ppm.