I know about the Co2 chart. I have checked that out previously.
I guess my concern is surrounding the fact that my tank has a Ungassed- No Co2 in it - ph of 7.5 and 10 degrees KH. These parameters tell me that I have co2 present in the tank, at about 10ppm with NO CO2.
fish give off "some" co2, but shouldn't be influencing it that much.
the 10KH giving 8pH is based on the atmospheric level of co2 being 3ppm, so it is right on target for your tap water results.
Also, what is your fish load like? As you feed fish and they produce waste, the acids will eat away at your KH a little bit. This is one of the main reasons for having a KH of above 4 in a non-planted tank. By saying that your pH is 7.5, that would imply a KH of 4 though, which is very doubtful that other factors are pushing the KH down. And you said it is still 10 KH anyway upon testing.
How sure are you that your KH levels are not dropping?