Total hardness (TH), sometimes called calcium hardness, is a measurement of the mineral content in your water. Total hardness should be kept between 150 ppm to 399 ppm.
Low Total Hardness, less than 150 ppm
Low total hardness can cause damage your pool walls and corrosion of metal surfaces like pipes, heaters, and ladders. To raise total hardness you'll need to purchase a calcium hardness increaser and carefully follow the dosage instructions to reach 200 ppm.
High Total Hardness, greater than 399 ppm
High total hardness can cause abnormal cloudiness and the formation of scale. Lowering total hardness can be difficult if the hardness is coming from your tap water. If this is the case, dilution will make no difference because the hardness of the new water will be the same as the old water. If you need to lower your total hardness it's best to speak with a local pool supply retailer. They may recommend the use of a flocculant to clump the minerals so that they can be removed from your pool.