General hardness (GH) is commonly overlooked in many aquariums, but this parameter plays a very important role in your aquariums overall health. GH is the total mineral content of calcium and magnesium in the water. These are important for various biological functions in fish, invertebrates, and even plants. Deficiencies of calcium and magnesium can cause poor growth and faded colors in fish, molting difficulties in shrimp, improper leaf development on plants, or thin, brittle snail shells. 

Freshwater systems will typically have a GH of 4-8 dGH (70-140 ppm). Soft water species like Discus prefer very soft water below 4 dGH (<70 ppm), while some African Cichlid Species prefer a GH above 8 dGH (>140 ppm).