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I posted this already two days ago, but there came no response, so I'll try

I am having problems determining my exact CO2-levels. I get different

First of all, at the time of the measurement, my tank had a pH of 6.84
(measured by pH-elektrode calibrated only a few days ago) and a KH of 4.0 as
measured by the Tetra KH test kit (instead of the recommended 5 ml of water,
I used 10 ml of tank water, so the KH should be accurate within 0.5 units).
The tank temperature was 25.4 C (measured by a temperature sensor). There
are no plants and fish in the tank yet because I first want to have all
parameters right before I introduce any plants. So there's only water and
gravel (which is completely neutral).

Now, here goes:

(1) Judging from the pH, KH and CO2 chart, my CO2-levels are in a range
between 17 and 19 mg/l. Right?

(2) However, according to the Tetra CO2 test kit, my CO2-levels are between
34 and 38 mg/l. (For those who don't already know, the Tetra CO2 test kit
works as follows: 20 ml of tank water, 5 drops of test reagent 1 and then
count the number of drops of reagent 2 until there is a slight pinkish
coloration. To
determine the actual CO2-level, one has to double the number of drops of
test reagent 2. In my case resulting in a CO2-concentration of 34 - 38
mg/l.(17 to 19 drops of reagent 2))

So my CO2-levels indicated by the tetra test kit are twice as high as those
indicated by the pH, KH and CO2 relationship. What can account for this big
difference between these two testing methods? Which one is right? What are
the exact CO2-levels in my tank?

Is it possbile that the pH, KH and CO2 relationship is not valid in my tank
because of the presence of other buffers than just carbonates and
bicarbonates? I don't know which others buffers that would be since I added
nothing to the tank water yet, not even fertilizers... My phosphate-levels
are approx. 0.2 mg/l according to a Dupla test kit. Is this enough to render
the pH, KH and CO2 relationship unvalid?

Any thoughts, anyone?

Davy Cleys