I get what your saying, the co2 tablet is not a permenant solution but more of a trail run to see how good it works. Test the water by taking 2ml of water and add 2 drops of a drop checker reagent (Dupla permanent CO2 Test) before adding a tablet see if its blue, green or yellow. Add a tablet wait a few hours and test the water again if green, hooray, if not add excel. I know it not accurate, but that the only test reagent I got.
Just an aside: Excel will not show up on pH test kits.
Also, one of the most important factors about CO2 is that its levels are stable, something that is not easily achievable with CO2 tablets.