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Re: Dissolved CO2 Question

On Tue, 22 Dec 1998, Sean wrote:

> I have a question about dissolved CO2.  I have a heavily planted 30 gal
> aquarium with fluorite in the substrate and 60W of light.  I had a diy CO2 set
> up into the filter intake.  My KH is 8 and a pH of 7.0.  I checked the
> dissolved CO2 (test kit from Red Sea) and it said it was 40 ppm, I checked my
> 20 gal tank that has a slower bubble count and it was 30 ppm (also heavily
> planted with the same conditions).  Finally I checked a newly set up tank
> (about a week old) with nothing in it but gravel and it read 16 ppm.  My water
> is a mix of 2/3 RO and 1/3 well water (yes, it is tough with a well).  Is this
> possible or is the test kit wrong?

In the case of the 30 gallon it might be possible, but there's no reason
that the new unoccupied 20 gallon tank should contain 16 mg/l CO2 after
a reasonable period of exposure to air, so I suspect the test kit might be
producing inaccurate results.

I'm not positive how the CO2 test kits work, but I suspect they're just an
acidity titration.  If so, then any compound that will lose a hydrogen ion
between the pH of your water and a pH of 8.3 (the endpoint of an acidity
titration) will appear as CO2.  Your well water may carry something (e.g.,
phosphate or organic acids) that fit that description.

> BTW, the thirty has 4 otos and 6 cory's
> all doing fine.  The two twenties have no fish.  My chart of KH to pH says it
> can be a maximum of 24 ppm.

Under the right conditions, you might be able to read the CO2 content of
a well-aerated sample of the RO+well water mix and substract that
"apparent" CO2 value from your other readings to correct for the
interference.  It may be a coincidence, but if you subtract the 16 ppm in
your new tank from the 40 ppm in your 30 gallon tank, the difference (24
ppm) is consistent with your maximum value from the KH - pH chart.  If you
do the same thing with your 30 ppm reading from the first 20 gallon tank
then you would conclude that it should contain something like 14 ppm of

All in all, it sounds like your better off using the KH-pH chart than the
CO2 test kit.

Roger Miller