How can it not effect the reading?? When I set up my co2 added tank my tap water was pH 7.4 and KH5 from the tap. However, in testing other tanks, I had noticed that my KH had a tendencey to drop over time to about 2. This despite doing weekly 40% (on average) water changes. I was concerned about this because I though that adding co2 would be more likely to result in a pH crash.
So I added about 3/4 of a cup of crushed coral to my Eheim about a week or two before hooking up the co2 tank. The effect was to raise the KH 2 dgs but the pH barely moved. Once the co2 was on the tank I began testing the pH to determine the effects of the gas and to determine if I needed to do anything about nightime levels.
The upshot was at the rate I ran the co2, I didn't have much to worry about. The tank thrived and the plants pearled. However, when I tried to test the co2 levels via the pH/KH method, it showed co3 levels to be much lower than would be required to keep the plants I had growing well and pearling the way they did.
The only explanation I could come up with for the disparity between the "purported" low co2 level and the actual plant health and growth was that the pH/KH method was not reliable due to the crushed coral in the filter distorting the readings.
I have long since stopped adding the crushed coral once I understood that the risk of a pH crash was remote at best.
However, my understanding of the nature of KH comes from the following from the FINS site:
Quote:
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Buffering capacity refers to water's ability to keep the pH stable as acids or bases are added. pH and buffering capacity are intertwined with one another; although one might think that adding equal volumes of an acid and neutral water would result in a pH halfway in between, this rarely happens in practice. If the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH. Conceptually, a buffer acts somewhat like a large sponge. As more acid is added, the ``sponge'' absorbs the acid without changing the pH much. The "sponge's'' capacity is limited however; once the buffering capacity is used up, the pH changes more rapidly as acids are added.
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I also though that the pH drop from adding co2 is due to the fact that some of it is converted to carbonic acid and it is this acid that lowers the pH. Since the pH/KH method of measuring co2 levels is based on the absolute values, why would something that has the opposite effect of co2 on pH and KH not distort the levels and thus make the test inaccurate?