Kh is a measure of total alkalinity. That is the measure of alkalinity as contributed by OH- and CO3-2 ions and alkalinity contributed by HCO3- ions.
Below a ph of 8.3 there is no CO3-2 ions present in a solution of water all the CO3-2 will convert to HCO3- and OH- ions too donâ€™t exist below a pH of 8.3.
Normal FW aquariums do not have a pH above 8.3. So all the alkalinity present is due to HCO3- ions only and this is what is the buffering capacity.
The above chart will give a wrong reading if you use phosphoric acid etc., or black water extract to buffer your water. It works great with non buffered water.
Read up the other connecting threads in the krib archives. Theyâ€™ll clear all your doubts regarding Kh-Gh-pH.
Gh by the way is a measure of water hardness due to Ca++ and Mg++ ions only. This has more to do with scaling of water heaters and Soap lather and not much help in a planted aquarium. Needless to say higher your GH higher will be the Kh.
As long as your pH is below 8.3 all the CO3-2 will be as HCO3- and you donâ€™t have to worry. The hardness component due to Mg does not affect planted aquaria significantly unless it is very very low. A Gh of 8 and above is good enough to supply Ca and Mg to your plants.
The above was in reply to a similar question in another forum.... I think this is also copied on IAH, I remember doing that a while ago.
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