Well, yes and no. There may be several ways to measure carbon dioxide
concentration, but the very common approach is to use a pH probe separated
from the measured solution by a gas permeable membrane. The CO2 diffuses
from the sample to the inner solution and changes its pH. This change is
then measured with the pH probe. Unfortunately this type of CO2 sensor
suffers serious interferences from NH3 and volatile amines, which are both
likely to appear in the fish tank water.

Regarding the pH probe impedance - it depends widely on temperature (and
the glass type and thickness as well). At room temperature some 100MOhms
sound reasonable, at 212F it can drop downto 1MOhm, but at 32F it can reach
1GOhm.

Josef

>Perhaps a better approach to approximating pH would be to
>measure dissolved carbon dioxide.  Evaporation is a big
>problem in vessels open to the atmosphere--as water
>evaporates, salt concentrates, and the conductivity increases
>independent of pH changes.  A similar problem occurs in
>measuring soil moisture.  However, dissolved carbon dioxide
>is generally present at pH levels between abut 5.6 and 8.5,
>which is fine for aquariums and pools.  Because carbon
>dioxide dissociates to form carbonic acid, which is an acid,
>the higher the CO2, the lower the pH.  This is independent of
>conducitivity.  
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