A pH drop from 7 to 6 in couple of days indicates a tenfold increase in the acid concentration
Correct. A 1.0 pH drop is a tenfold increase in acidity since pH is a logarithmic (log10) scale. But all the rest of your reasoning is unfortunately wrong.
And the only source of acid I can think of is the CO2.
No. The acidity will increase as CO2 is injected due to the formation of carbonic acid. But there are multiple chemical processes in tank that are constantly changing the pH. Just to name a few: mineral content of rocks, wood, soil, plant photosynthesis and respiration, livestock respiration, fertilizers, food, bacterial activity, nitrification processes, ... This means that we cannot rely on the pH of the tank water to determine CO2 concentration since we cannot factor in the effect of all these processes.
I use the PH/KH tables to measure CO2, as I don't know any other way to do it.
The pH/KH tables are only applicable to controlled water conditions since they are calculated according to chemical formulas that make assumptions about the water parameters. These assumptions are not applicable to an aquarium or any other natural environment. You cannot even properly measure the KH of your tank without specialized equipment (the KH test kits are measuring much more than KH). What the pH/KH/CO2 tables is telling you is absolutely meaningless.
The drop checker is just a PH-test, doesn't give you any CO2 readings.
Yes, it is only a pH test. But it gives you an accurate reading of the CO2 concertation in the water.
The drop checker is not measuring the pH of the tank water but the pH of a calibrated solution (usually KH = 4° dH) inside the drop checker. The acidity of this solution will change according to the amount of CO2 in the water that will degass into the air gap inside drop checker. This means that the colour of the drop checker only changes as a result of changes to the CO2 concentration dissolved in the water. In contrast, the pH of the tank water will change not only because of injected CO2 but also because of dozens of other process that we are not controlling.
The issue with the drop check is that it is a colour-based test that is hard to read precisely, its reading depends on the location in the tank, and takes a very long time to react to changes in CO2. Even with these limitations, the drop checker provides a precise measurement of CO2 concentration. The drop checker is actually the only CO2 measurement you can use...
Read a <
summary of the problem here>, and a <
more authoritative sticky thread here>. There are dozens of other threads discussing this same topic at UKAPS since it is a recurring topic, often fueled by these silly pH/KH tables...