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Julius Odian wrote:  "The main thrust of my comment was geared at the fact
that in my opinion, it is not a good idea to use CO2 as a means to
artificially lower the PH of the water below it's normal equilibrium level.
Effects could be a rapid rise in PH in the event of a power outage on a
solenoid system or a rapid rise in the event of the CO2 bottle/source
becoming empty."

Let's look at the implications of Julius' first sentence.  The equilibrium
level of dissolved CO2 in a water column exposed to air is fairly low,
around 2 - 3 ppm CO2.  This is well-below the optimum level for growing many
species of aquatic plants.  We might see a general increase in plant
vitality at say, 10 - 15 ppm and a variety of plants seem to prefer 20 - 25
ppm CO2.  If we are not to artificially lower the pH by adding CO2, then we
must not add ANY CO2 to the water column.  Because as we know, adding CO2
will in fact lower the pH.  I wonder if Julius is recommending not adding
any CO2 to our tanks to avoid potential fluctuation in pH or perhaps
something else, such as not adding much CO2 to the water column.

I think many of us have observed that occasional fluctuation in the pH does
not seem to affect fish as long as it happens slowly.  Others have reported
that natural waters have considerable daily fluctuations in pH.  Those who
use a solenoid to shut the CO2 off at night also have significant daily
fluctuations in pH and I am not aware of any reported problems.  So in
general I would conclude that the risk of pH fluctuations associated with
the use of CO2 is not so extreme as to preclude us from using CO2.  My own
view is that we ought to take steps to moderate the "dumping" affect of most
of our hobby CO2 equipment as the bottle runs out of gas.  Even this
phenomenon has not caused difficulties that I am aware of in my tanks.  As I
have mentioned I use a Victor regulator and Nupro M series fine metering

I made a mistake in a recent post which I also made last year which Paul
Sears was kind enough to correct at that time.  For some reason of other I
seem to think (or at least report! :-)) that pH fluctuation is partly a
function of KH levels.  It is not and one can see this by looking at the
chart and comparing different pHs at different CO2 levels and also at
varying KH levels.  My apologies.

Regards, Steve Dixon in San Francisco where summer continues