Hi all,
I'm not a CO2 user, but if I was I would use a drop checker, purely because the 4dKH solution you use in the drop checker
only has carbonate hardness, and no other source of hardness (OH- etc.).
The only acid that enters the drop checker is the carbonic acid (H2CO3) from the dissolution of (a small proportion) of the added CO2, that diffuses across the air gap, along the CO2 concentration gradient, into the drop checker.
The Red Sea KH / Alkalinity Pro Test Kit
It is a bit different in sea water, the water is highly carbonate buffered, and always alkaline. Because sea water
is <"very highly buffered"> the pH is stable at ~pH8.2 and it is easier to separate the alkalinity due carbonates (buffering at pH8.2), from those caused by other bases (which will drive the pH higher).
The pH8.2 is important because carbonates are insoluble in water, but soluble in weak acids (like the carbonic acid from the dissolution of CO2) and at atmospheric CO2 levels (400 ppm) water with any carbonate buffering (dKH) will have a pH of pH8.2. If you raise the level of CO2 (by adding it) the carbonate ~ CO2 equilibrium is driven towards H2CO3 and the pH falls, but this doesn't change the alkalinity.
Additionally marine aquarists have a special problem because they have molluscs, corals and coralline algae that actively deplete the both calcium and carbonate from the water. it is slightly different for freshwater aquarists because we might have snails, but we don't have other macro-organisms that deplete hardness (although microbes will).
cheers Darrel