Tetra Test GH is a kit used for determining GH (general hardness) levels. It is a liquid kit， which works by changing the color of a water sample regarding to the GH concentration. You just simply need to count the number of drops it takes to turn the water sample from red to green in order to determine the GH level. We recommend testing for GH once a week in soft water areas， and once every 2-3 weeks in hard water areas. Always read the instructions carefully before using. If GH levels are unsatisfactory then you should seek advice as to the underlying cause.