The General Hardness (GH) of water has a direct influence on the regulatory functions of fish (fish skin, gills, and eggs), plants and microorganisms. A favorable GH value for most ornamental fish between 6 and 16 dH (degrees of General hardness). Often forgotten or lost in significance to testing pH, ammonia, and nitrite, General Hardness is vital to selecting fish suitable to local water supplies or testing the performance of advanced filtration methods including reverse osmosis or deionization. This kit contains one 10-ml test reagent, a test vial and instructions and recommendations. This kit is for Freshwater aquariums only.