No, I'm not saying CO2 does anything to KH.

I am saying the referenced KH you added will diffuse out, just like the CO2 diffuses in.

That's how these devices work.

Use the tank's water, not a referenced distilled water and baking soda.

The water inside these checkers will equilibrate with the tank's water.
So the KH and other things in the tank will influence the checker over a few hours/day or so etc.

The CO2 has to get in there to change the color and show that there is a change in CO2 through the day right?

Same deal here, except in their case, the issue is KH changing to what is inside the tank and diffusing out of whatever small referenced KH solution you added to the checker.

Not sure if this is clear enough.

You are not adding KH.
You are adding CO2.

The KH will change becuase of the tank's KH and other ions will diffuse into the checker. This will not take that long.

Then that reference solution in the check will show a different reading.

I think the idea is good initially, but the reality is that the the KH/checker solution will change rapidly and you may as well just add tank water, not distilled and baking soda etc.

Regards,
Tom Barr