High pH with low kH

[RickyV]

What would cause a high pH with a low kH? My tank has a 2.5 kH with a 7.8-7.9 pH. I use a milwaukee pH probe to test pH. KH was tested with a salifert test kit. The water source is RO water but the tank has concrete rock structures that leak calcium carbonate during CO2 injection. Does calcium carbonate just raise the pH disproportionately compared to the kH? Or my degasssed holds very little CO2 for some reason? Or is this standard pH for the kH? My TDS is 170 ppm. GH 7

 

[hax47]

Those pH values correspond to approximately 1.6 - 2 ppm of dissolved CO2. These are not unrealistic values, depending on your tank system, how much CO2 is produced, how much is used, what the indoor air-CO2 is, how good is the gas exchange between the air and water, etc.
So I guess the low CO2 would cause that high pH.
The calcium carbonate does not raise the pH disproportionally, the KH/CO2 ratio will determine the pH.

 

[Zeus.]

As you decrease the pH certain rocks will release Ions and Cations which in turn can change the KH and GH even when RO water is used. @Wookii had this issue some time back.
I assume that the concrete rock structures are doing the same.
When trying to maintain specific KH and GH when using RO water it is always best practice to use inert rocks