Carbonate hardness, or alkalinity (KH), is the measure of both carbonates and bicarbonates present in the water. The KH value of your water is important to know, as the carbonate hardness plays a significant role in the water's pH and the water's buffering capacity to keep that pH stable. KH naturally falls over time with the introduction of CO2 and other acids, and nitrification. Reverse osmosis water should be buffered with carbonates before use to raise and maintain KH. Some municipal tap water can have variable KH levels throughout the year and should be checked regularly. In saltwater tanks, KH can be tested to monitor the proper ionic balance when dosing. Soft water fish, like Discus and other South American cichlids - 0 to 4 dKH (0 - 70 ppm) Tropical community fish, like tetras and betta - 4-8 dKH (70 - 140 ppm) African rift lake cichlids - 8 to 12 dKH (140 - 215 ppm) Marine species - 8 to 12 dKH (140 - 215 ppm)