But, 30 ppm of CO2 dissolved in water isn't in equilibrium with 30 ppm of CO2 in air. My education is lacking in this field, so I have no idea how to even start to solve this.
Air can have a varying amount of CO2 in it. If it is .03% CO2, it is 300 ppm, but water exposed to that air will contain more like 3 ppm of CO2. If the ppm in the water in equilibrium with the air above the water is a near constant percentage of the ppm in the air, this is an easy problem, but Murphy's Law seems to dictate that it isn't anywhere near a constant percentage.
So it is an issue with the rate of diffusion, but also the solubility of CO2 in the water?
I'm a microbiologist, not a chemist (though I have a minor, haha). Perhaps we need to get a chemist on this
Same reason not to to use tank water in a drop checker. The ph will be affected by other things besides co2 throughout the day like rocks, substrate, driftwood. We're not dealing with an empty tank of water.
Aha, I don't know why I missed this.