Excuse me, Phil; but kh measures alcalinity; therefore Kh indicates CO3=, HCO3-, (OH)-, .... anions in the water.(No only CO3=).
If you dissolves (by example) Ca(oh)2 in a destilated water cup then kh goes up. (there are´t CO3= anions in solution).
If you put amoniac in a glass of water Kh goes up (because NH4(OH) is formed)
A second thing: Carbonate calcium dissolves in water very bad; it goes in solution by first changing in calcium bicarbonate in the presence of carbonic acid. This soluble bicarbonate rises KH (no CO3=(aq))
In typical aquarium water the measurement of KH is produced by bicarbonates in solution.
Please,excuse me my english.
Regards, Juan Carlos.