The pH/KH/CO2 relationship is not valid in tank water which has other acidic components in the water affecting the pH. The relationship is only ever valid when the contribution to acidity is due to CO2 alone. A drop checker using distilled water, and which is absolutely free of any other acids has it's pH change ONLY as a direct result of the dissolved carbonic acid. If you use tank water in a drop checker then you may as well not use a drop checker at all since this sample water is corrupt. If you calculate CO2 by merely using the tank water pH as a basis for your calculation then whatever number you calculate for CO2 is also totally corrupt.

It therefore does not matter that ayhy2mark33's tank water is 7.1 in the morning, or that his pH is 6.6 later on. These measurements tell you absolutely nothing about the precise CO2 content. It only matters what the pH of the water in his drop checker is. After adding CO2 both the drop checker's sample water and his tank water will experience a fall in pH. The difference is that his drop checker water's pH will fall in proportion to the CO2 in accordance with the relationship, whereas the water in his tank will fall in accordance with some other relationship that is of no use to us.

Hope this helps to clarify.

Cheers,