Many aquarium owners think that the general hardness represents the total of all minerals and the carbonate hardness only a part of it. Then how can the carbonate hardness sometimes be higher than the total of minerals?
In a large number, indeed practically in all tropical waters, however we find just this situation: In Lake Malawi and Lake Tanganyika the KH lies significantly above the GH, and also in many rivers we can find a low KH, although a GH is not measurable. If you take a close look at the definition of the values, you will notice that the GH doesn’t represent the total of minerals but only the sum of calcium and magnesium. Actually it should be named Ca-Mg hardness! On the other hand the carbonate hardness indicates the quantity of carbonates (HCO3) and bicarbonates (HCO3)2. Non-chemist can’t be expected to understand what this means. But it’s a fact that both forms of carbonate are bound to metals, which normally would be calcium, magnesium, potassium or sodium. The KH doesn’t care what metal it is bound to. If bound to Ca or Mg, there is also a GH present. But if it is bound to K or Na there is a KH present, even if no GH components (Ca and Mg) are available. Since a lot of waters have no calcium and magnesium, their GH is not measurable. But there are often few quantities of carbonates present, which are bound to potassium or sodium. And then you can measure the KH but not the GH. Okay?
If not, just read these lines a few times again. It mostly becomes clearer then. It’s indeed a really interesting situation which explains the results of GH and KH tests with a basic knowledge of chemistry!
