[RTR]

TTA - you are working under and handicapped by unjustified assumptions. The pH/KH/CO2 test assumes that no other significant acid is present, and that is obviously not the case for many to most fish tanks.

This test is not a direct measure of CO2, It is a measure of the difference of the observed pH versus the expected pH from the pH expected at a given and immediately tested KH.
- You must have an immediate reading of the KH currently in the tank, not that of the tap water if you have modified the water in the tank (as by aragonite or crushed coral or bicarb or limestone addition). Nor can you guesstimate the KH from the post-partial KH observed at some other time, it is a now reading.

Tanks generate acids. These in practice tend to be fairly stable within some range dependent of the the frequency and percentage of the partials, the stocking, feeding, and all the usual suspects. But it is not a stable figure, it is a range. If you assume that your current acid levels are those observed immediately post partial, you CO2 reading will be off on the high side. If you assume that your acids are at immediately pre-partial readings, you CO2 reading will be off on the low side. If you assume they are always trivial, your readings for CO2 are meaningless, just as Bunny13's tests have shown in this thread.

Always examine the assumptions. Hobbyists generally have a really hard time with that one, and it is an honest mistake. They are told all sorts of things which are valid under certain conditions, but very, very rarely told how to check those assumptions. On CO2 that is how and why so many hobbyists report sedative to potentially lethal CO2 levels which are observably not valid. A level of 30ppm CO2 is neither of those negatives, but it quite different in results from 12 ppm CO2, which turns out to be closer to the fact in this thread.

HTH