GH is an abbreviation for General Hardness, and it's a reference to how hard or soft your water is. If freshwater contains lots of dissolved minerals it is described as hard, and if it is low in minerals or devoid of minerals altogether it is known as soft.

Seawater contains huge amounts of dissolved minerals, so is always hard. Reverse Osmosis water has been stripped of minerals, so is always soft. Limescale on taps and in kettles is always an indicator of hard water.

What’s the difference between GH and KH?

KH is an abbreviation of the German Karbonat Hardness, and we refer to it as carbonate hardness, or alkalinity. KH refers to how many carbonates are in the water, where as GH refers specifically to calcium and magnesium. In general, hard water has a high pH, KH and GH, but not always, and tap water with a low pH but high GH is not impossible, or high pH and GH but low KH!

Hard, alkaline water is the opposite of soft, acidic water. Popular soft water fish include Discus and Cardinal tetras. Popular hard water fish species include Malawi and Tanganyika cichlids, so if your tap water has a high GH and you want to keep soft water fish, you need to lower it. 

If you require a low GH in your tropical aquarium, the first thing to do is to make sure that you have no calcareous rocks or gravel like coral sand or Ocean Rock, which contain elements which raise and buffer GH. Make sure that you only select inert, lime-free gravel and rocks when setting up the aquarium.

The best way to introduce water with a low GH is to fill the aquarium with Reverse Osmosis water. This tap water purification process removes nitrates, phosphates, chlorine and minerals, and produces water that is pure. Treat the pure RO with a remineralizer for RO and you will have perfect aquarium water.

If you have an existing aquarium with high GH and you want to lower it, you can start to introduce RO water in your next water change. Neat RO can be used in conjunction with a GH test kit to slowly bring GH down to required levels.

If your tap water has a high GH then RO is the only realistic way to lower tank GH, as it is not as easy to make hard water soft, by other means. Avoid products which lower pH or KH as it is not the same and won’t remove the calcium and magnesium from the water.

If your tap water has low GH but your tank water has a high GH, first check that there is nothing buffering GH, like calcareous decor, then use dechlorinated tap water in water changes to bring GH down. Rainwater has a low GH, although it may bring pollutants with it. 

Which GH values are hard and soft?

GH is either measured in dH (degrees hardness,) or ppm (parts per million.) 0-4 °dH (0-70 ppm) is very soft, 4-8 °dH (70-140 ppm) is soft, 8-12 °dH (140-210 ppm) is medium, 12-18 °dH (210-320 ppm) is hard and 18-30 °dH (320-530 ppm) is considered very hard.

Look for a freshwater aquarium test kit that tests for pH, KH and GH in one comprehensive kit.