CO2 Measurement

Thanks to everyone who commented about their favorite CO2 test kit.  To 
summarize, the LaMotte kit was widely thought to be reliable, but nobody 
liked the Tetra kit.  Nobody, not even George "I have money to burn" 
Booth ;-), mentioned the Dupla kit.  Both Earle Hamilton and Karen 
Randall suggested that ultimately it may not be necessary to know CO2 
concentrations with great precision.

Perhaps it would be useful to describe why I was interested in the
kH/pH/CO2 question.  My 75 G planted tank has been running for about 4
months.  It is a DIY high tech system - under gravel cables (currently off
most of the time because of the hot Houston summer), manual CO2 injection
(CO2 bubbled into powerhead intake and mixed in a DIY reactor), adequate
light (> 2 watts/g) and Dupla fertilizer and drops. The tank inhabitants
are mainly rainbowfish, and because of this I felt constrained to keep the
pH around 6.9-7, since rainbows do not appreciate an acidic environment. 
The water has a kH of 4-5, so with a pH of around 7 this translated to
approx 12-15 ppm of CO2, which is pretty much optimal. The plant growth
was fairly mediocre, and not at all what I had hoped for.  It seemed that
I was doing all the things recommended by net wisdom, but something was
obviously missing.  This was why the lack of correlation between kH/pH and
CO2 in Amano's data caught my attention, since CO2 was the only thing in
my tank I was not measuring directly.  After George pointed out the
potential pitfalls in guesstimating CO2 from the table, I decided to
become a more aggressive experimenter.  I found that I could more than
double the rate at which I was adding CO2 to the tank and the pH would
remain at 7.0 while the lights were on.  I interpret this to mean that the
plants are absorbing the CO2 as fast as I can add it.  I was concerned
that this higher CO2 input would cause the pH to become too low at night
when the plants are net producers of CO2.  To offset this I decided that
after lights out I would turn off the powerhead that drives the CO2
reactor.  CO2 input would remain the same, but mixing would be much less
efficient. I thought this might cause the pH to rise a little, but for the
past 2 days the pH in the morning has been around 6.9, which is fine.  I
think my next step will be to wire the powerhead into the same timer that
operates the lights, so that I will have a semi-controlled CO2 system.  I
am cautiously optimistic that this increased CO2 will do the trick - the
plants are producing more O2 and are showing definite signs of new growth. 

While I agree with Karen that measuring CO2 may not be an essential for 
experienced aquatic gardeners, I think I might have solved this problem a 
bit quicker if I had been making direct measurements instead of using the 

On a related topic, John Ching wrote:

>When I first mix the water with the reactant by lightly shaking the vial
>for 5 seconds, I get a reading of 6.4. If I continue to shake the vial
>lightly for another 20-30 seconds, the pH reading goes up to 7.0.  Now
>which reading is the correct one. 

The first reading is correct.  As you surmised, the more you shake the
vial, the more out gassing of CO2 will occur, and the higher the pH will
become.  After you add the indicator dye, just invert the vial a couple of
times. This should be sufficient to mix the dye evenly with the water.