Did you aerate your replacement water?
The CO2 in a Tank without injection should be around only between .07mg/l(ppm) to .6 mg/l (ppm), depend of surface agitation.
I think the gasping is probably due to low oxygen in the water...
—> do you have lot of water movment
—> do you have pearling of your plant
—> what type of filtration
The Chart become unusable in presence of any type of buffer, acid (peat, ph minus etc or alcali (CACO3, ph +)
You have to use Krause system to find your co2 content :
Hanns-J Krause CO2 Test
Author Handbook Aquaria Technology
Krause recommends a different method to measure CO2 content. That method is insensitive to the presence of unusual buffers.
Step 1: Take another water sample with some pH indicator and run the hose from an air pump into the sampe for a few minutes. This sets the CO2 level at 0.6ppm.
—>Measure pH of the water (=X).
Step 2: Take small water sample with some pH indicator. Stick a straw into the water and exhale through the straw into the water sample for two or three minutes. This sets the CO2 level at 60ppm.
—>Measure pH of the water (=Y)
The optimum CO2 concentration of 10-20ppm is at the pH value about 2/3 of the difference between X and Y:
pH,opt = X +.67*(Y-X).
As an example, if you measure pH 8 with the first sample, and pH 6.5 with the second sample, a tank pH of 7 corresponds to 30ppm CO2, regardless of any buffers that might otherwise confuse a CO2 test or distort the charted values.
PH Opti (6.995) = x(6)*(y(8)-X(6)