@twentyleagues - I will try to clear up the doubts regarding the very German term water hardness. There are two terms KH and GH
KH stands for Karbonathärte (carbonate hardness) and GH for Gemeinsamkeit Härte (common hardness). KH is a measure of the carbonate ion content of water and GH includes many ions including calcium and magnesium which means it is completely pointless to measure in salt water which is very rich in both calcium and magnesium.
In salt water the alkalinity consists mostly (normally more than 96%) of bi and carbonate ions and therefore the German term KH became more or less a measure of the total alkalinity (both in salt and fresh water). The concept 7 in KH should actually be written 7 °dKH which stands for 7 degrees Deutsche Karbonathärte since there are many national scales for carbonate hardness. This aquarist concept dKH is valid for both salt and fresh water. In science, other concepts than dKH are used - there, alkalinity is calculated in mg/L CaCO3 or in mEq per liter. The conversion is 1 dKH = approx. 17.857 mg/L CaCO3 = 2.8 mEq/L. 1 mEq/l = 1 mmol/L HCO3
@Randy Holmes-Farley - correct me if I´m wrong or express myself badly (using swenglish)
Sincerely Lasse