http://www.drhelm.com/aquarium/chemistry.html
kH
Carbonate hardness or temporary hardness. Measures the buffering capacity or the ability to absorb and neutralize added acid without major changes to pH.
gH
General hardness (GH) refers to the dissolved concentration primarily of magnesium and calcium ions. Other ions can contribute to water hardness but are usually insignificant and difficult to measure.
When fish are said to prefer ``soft'' or ``hard'' water, it is gH, not kH that is being referred to. gH will not directly affect pH although "hard" water is generally alkaline due to some interaction of gH and kH.
http://www.sera.de/English/autoframe.ht ... H-test.htm