10-02-2020, 17:27
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GH is an abbreviation for General Hardness and indicates how hard or soft your water is. If fresh water contains a lot of dissolved minerals, it is called hard water and if it has few or no minerals, it is called soft water.

Seawater has a large quantity of dissolved minerals and is therefore always hard. Reverse osmosis water is free from minerals and is therefore always soft. Lime scale on taps and kettles is always an indication of hard water.

What is the difference between GH and KH?

KH is an abbreviation for German hardness by carbonate and we call it carbonate hardness or alkalinity. KH refers to the amount of carbonate in the water, and GH specifically refers to calcium and magnesium. Typically, hard water has a high pH, KH and GH, but this is not always true. Tap water can have low pH and high GH, or high pH and GH but low KH.

Soft water is required for Discus and Tetra, while hard water is required for Malawi and Tanganyika cichlids. Therefore, if the tap water has a high hardness and you wish to keep fish that require soft water, the hardness of the tap water must be reduced.

If you want low water hardness in the aquarium, the first thing to do is to make sure there are no lime stones or coral sand in the aquarium, which contain elements that increase water hardness. Make sure that you only use inert, lime-free gravel and stones.

The best way to get low GH water is to fill your aquarium with reverse osmosis water. This treatment removes nitrate, phosphate, chlorine and minerals from tap water. Also note that preparations that lower pH or KH do not remove calcium and magnesium from the water.

If the tap water is soft, but the aquarium water is hard, remove all lime décor from the bottom and then carry out systematic water changes with pre-calcified tap water to lower the water hardness. Alternatively, distilled water can be used to lower DH by replacing part of the aquarium water. Do not use rainwater for the aquarium, as it often contains a large number of harmful impurities.

Which GH values relate to hard and soft water?

GH is measured either in dH (degrees of hardness) or in parts per million (ppm):

  • 0-4° dH (0-70 ppm) - very soft water;
  • 4-8° dH (70-140 ppm) - soft water;
  • 8-12° dH (140-210 ppm) - water of medium hardness;
  • 12-18° dH (210-320 ppm) - hard water;
  • 18-30° dH (320-530 ppm) - very hard water.

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