Iain Miller wrote in message
...

CO2 does NOT affect KH.

Beg to differ. This is quoted from RedSea:

"CO2 gas dissolved in water can take the form of bicarbonate or carbonate.
These three forms are in chemical equilibrium. Which form is present in what
amount depends on pH.

Dissolved CO2 = bicarbonate ion = carbonate ion

In the pH range favorable to water plants, pH 6.4 - 7.2, a percentage will
be present as dissolved CO2 and the rest as bicarbonate ions.
From pH 8.0 to pH 8.8 there will be almost no dissolved CO2, a large amount
of bicarbonate and a small amount of carbonate."

From which I conclude that carbonate hardness can be reduced by lowering
either CO2 or pH.