GH is a measurement that indicates the amount of dissolved minerals like calcium and magnesium. These minerals are important for good growth of water plants and also fish health. Optimal plant growth is also excellent for blocking growth of unsightly algae.
The best GH value in freshwater is between 8 and 12, when it is above 15 it is too hard and white lime deposits can occur. This can be damaging to pumps and internal filters as deposits may cause the pump or filter to cease.
Full testing instructions included in the box.