JBL ProAqua Test GH General Hardness for the quick, reliable and accurate determination of the total hardness in fresh water and garden ponds. It corresponds thereby to 1 drop = 1dGH. Colour change from red to green.Why test? Depending on its origin, and often due to the nature of the subsoil, water can contain different amounts of different mineral salts. By definition, general hardness is the total concentration of all alkaline earth ions in water. The general hardness is mostly formed by calcium and magnesium salts. Most fish and plants can be kept successfully with a general hardness of about 8-25dH. In garden ponds, low values for the general hardness are often measured and are due to the dilution caused by precipitationYou can oder the complete test kit with coloure chart and test accessories or only the reagents at the refill kit.