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What is my CO2-concentration???

I am having a bit of a problem determining my CO2-levels... I get some
contradictory levels when I try to determine my CO2-concentration using
various methods...

First of all, at the time of my measurement, my tank had a pH of 6.84 as
measured by an 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 5 ml of water, I used
10 ml of tank water to measure the KH, so it should be accurate to 0.5
units). The tank temperature was 25.4 C as measured by the temperature
sensor. There are no plants and fish in the tank yet, just water and gravel
(which is completely neutral)...

Now, here goes:

(1) According to the pH / KH / CO2 chart, my CO2-levels are between 17 and
19 mg/l.

(2) However, according to the Tetra CO2 test kit, I have a CO2-concentration
between 34 and 38 mg/l. The Tetra CO2 test kit works a follows: 20 ml of
tank water, 5 drops of test reagent 1 and then, you just have to count the
number of drops of reagent 2 until there is a pinkish coloration. To
determine the actual CO2-level, one has to double the number of drops of
test reagent 2. I needed 17 to 19 drops to get a pinish coloration of the
sample, which is equivalent to a CO2-concentration of 34 - 38 mg/l.

According to the Tetra test kit, my CO2-levels are twice as high as the pH /
KH / CO2 chart indicates...

(3) According to the Dupla CO2 Dauertest, I have an optimal CO2-level since
the test sample has a green colour... (blue = too low, green = optimal and
yellow = too high).

So can anyone tell me what my exact CO2-levels are? I am a bit confused

Why is there such a big difference between the results of the Tetra CO2 test
kit and the CO2-level indicated by the pH / KH / CO2 chart?
Maybe the pH / KH / CO2 relationship is not valid in my tank since there are
other buffers present? I wouldn't know which ones, since I added nothing to
the tank water yet, not even fertilizers... Furthermore, my phospate levels
are approx. 0.2 mg/l as indicated by the Dupla PO4 test kit. Is this enough
to render the relationship unvalid?

Any thoughts, anyone?

Davy Cleys
Kouterstraat 11
B-9150 Rupelmonde
Tel. +32 (0) 3/774.07.95
GSM 0498/61.20.04
E-mail: davy.cleys at skynet_be