Do I Need CO2???

To: Aquatic-Plants at actwin_com
Subject: Do I Need CO2???
From: George Booth <booth at hpmtlgb1_lvld.hp.com>
Date: Tue, 18 Jun 1996 09:18:52 -0600
In-Reply-To: Your message of "Tue, 18 Jun 1996 03:39:02 EDT." <199606180739.DAA15261 at looney_actwin.com>

> From: John Lobingier <jlob at wpa_net>
> Date: Tue, 18 Jun 1996 05:02:14 GMT
> 
> According to my test results and the chart in "The Optimum Aquarium"
> I do not need it.
> 
> My pH is 6.0 in the tank.  Using a Tetra test kit I got a reading of
> 1dh of KH and 6dh of GH.  I got the same readings from the tank and
> straight tap water when testing for hardness.  According to the chart
> in "The Optimum Aquarium" which determines CO2 levels according to pH
> and KH readings I have a CO2 level of 30mg/l.  Is this chart accurate?

The chart is accurate.  How about your measurements? 

You are assuming that your measurements are exact.  The Tetra test kit
is rather granular.  Using 5 ml of water, you may be off by +/- 1
dKH.  Most colorimetric pH test kits are also rather coarse,
especially at their far ranges.  Your pH may be 5.8 or 6.4.  Given a
possible range of measurement error, your CO2 levels may be anywhere
from 3.7 ppm to 70 ppm.  

Note also that the "KH" test is not specific for "carbonate hardness"
but is measuring alkalinity.  If carbonate hardness is the only source
of alkalinity in your water, you KH measurement is usable with the
chart.  If other things contribute to alkalinity, the "KH" measurement
should be taken with a grain of salt (or other base :-).  The chart
assumes your KH measurement is purely carbonate hardness.

Also, if your water has very little buffering (which is indicated by a
KH of 1), pH will be greatly affected by other acids in the water
besides carbonic acid (which is equilibirium with dissovled CO2).  

Both these conditions would lead me to distrust the use of a pH/KH/CO2
chart in your situation. 

> For the first time I bought an alkalinity test kit and got a reading
> of 10 for alkalinity/buffering capacity.

"10" what?  degrees KH or ppm (most likely).  10 ppm is very soft and
could be boosted with sodium bicarbonate to increase your buffering.   

I would suggest worrying about other aspects of water chemistry before
playing with CO2 injection.  With your current setup, CO2 injection
would produce dangerous pH swings. 

George