Perhaps I did not state clearly what I did. I knew I was about to set up a new tank that would be using pressurized co2 and that my tap water had a KH of about 5 dg. I also knew I did a 35-40% weekly change on all my tanks and would do so on the co2 added one about to be set up. As a precaution I wanted to be sure of what might be going on with the water params in my existing tanks in regards to KH levels. So I tested. What I discovered was the tank KH levels were consistently lower than my tap- as low as 2 dg. This indicated to me that without adding any co2, my tank KH was dropping over time- for whatever reason.
Therefore, as a precautionary/preventative measure I wanted to see what I could do to keep the KH levels higher. I decided to try adding some crushed coral in a bag on one of the filters on a 45 gal tank (the new co2 added tank was to be a 50 gal and I figured the 45 was pretty close in volume and had similar gravel and wood). After adding the coral I began to test the kh and discovered it climbed gradually over the next 2 weeks by about 2 dgs. However, the pH remained stable. This lead me to believe that a small amount of crushed coral would up the KH but would barely move the pH at all. Any more coral and I would have expected also to get a bit of a pH bump.
The upshot was I decided to add a similar amount of crushed coral to the Eheim on the new tank to help prevent any chance of a pH crash due to adding the co2. Once the new tank was up and running, I began a period of regular testing of the pH to determine how much the addition of co2 effected it. Tests were done several times a day in order to determine what change was happening and to determine what need there might be for dealing with nightime params with the gas running 24/7. I was prepared to outgas via an airstone at night if the need for this was evident from the testing. At the bubble rate I was running, there was not a huge pH drop overnight and I decided to allow the system to run 24/7 without any check on nightime rates.
The next thing I wanted to determine was what level of co2 was in the tank at the bubble rate I was running. For this I decided to try the pH/KH testing method and the chart posted above. And that is when I began to see unusual readings. I had planted a number of light loving, co2 needing plants, ones which would not thrive if nutrients and co2 levels were insufficient. In typical English garden fashion, I had overloaded the tank with plants which included rotala macrandra, ludwigia glandulosa, didiplis diandra. These plants did great, colors were vibrant, growth was rapid and pearling was ongoing- at times the tank looked like somebody had dropped in a couple of Alka Seltzers the pearling was so robust:
But when I began to test the pH/KH to extrapolate co2 levels, my results were indicating levels of about 14-15 ppm. But I also knew that this was an insufficient level for the results I was getting in terms of how the plants did. I expected to see something more on the order of 25 ppm or so. This is what led me to conclude that the addition of the coral and its effect on KH was throwing off the readings for actual co2 levels in the tank.
All this aside, as both a land and aquatic gardener I have always tended to rely on how the plants are doing as a guide to what I needed to add in terms of ferts etc. rather than using more technical methods of testing. I am an eyeballer more than "scientist".