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Fw: [APD] Re: CO2 off at night

> Date: Thu, 11 Sep 2003 15:44:24 -0600
> From: gbooth at frii_com
> Subject: [APD] Re: CO2 off at night
> To: aquatic-plants at actwin_com
> John T. Fitch wrote:
> > I do not understand this explanation.  It seems to me to be the
> > of my understanding of what happens. When the lights go off, the
> > give off CO2 and the pH drops. If the CO2 is left on continuously, the
> pH
> > will drop even further. When the lights come back on, the plants start
> > using CO2 and the pH rises ... Is this not correct?
> Yes, that is not correct.
> When the lights go off, the plants stop using CO2 and the pH goes up as
> CO2 diffuses out of the tank.
> Plants always respire CO2, whether they are photosynthesizing or not.
> they are photosynthesizing, they also generate O2.
> I'm not sure if the plants respire more CO2 during the day than night or
> not. Do they sleep and slow down their internal processes?  I don't
know. But
> they don't respire more CO2 at night causing the pH to drop.
> The plants themselves don't generate enough CO2 to be useful for our
> purposes. Wouldn't it be wonderful if they did? Sort of like perpetual
motion. In a
> non- CO2 injected tank, typical CO2 levels are about 3-5 mg/l due to
> plants and bacteria respiration. With the CO2 off at night, CO2 will
diffuse out
> of the tank and get to that level by the morning (see my "CO2 Loss in a
> Trickle Filter" article on my website to see how quickly CO2 is lost
based on
> various factors other than the trickle filter itself).
> A further point is that most of the CO2 "usage" in the tank is diffusion
> to the atmosphere, especially at higher concentrations. I can't venture
a guess
> on exact ratios, but I would conjecture that _most_ of the CO2 we inject
> not get used by the plants.  This implies that when you turn on the CO2
in the
> morning, a lot of incoming CO2 is lost right away and it takes longer
> you think to get to useful concentrations.
> And, of course, the high tech wonder of a pH controller gets around all
> this complication by keeping the CO2 level pretty constant no matter
what the
> conditions. Not without it's own minor annoyances, of course.
> George Booth in Ft. Collins, CO (gbooth at frii dot com)
Sorry, but this is not what happens in my tank.  At night, when there is
no CO2 added to my tank, the pH does *not* go "up as CO2 diffuses out of
the tank."  The pH goes down from 6.9 (my controller set point) to 6.7 or
6.6.  How would this be possible if the *net* flow of CO2 is out of the

In the morning, when the lights come on, the pH level slowly rises over an
hour or two back up to 6.9.  At that point, the contoller turns the CO2 on
and off for the rest of the day. This keeps the pH from rising any
further, and hence the CO2 concentration from dropping any further, during
the photoperiod.

Earlier, I tried to explain what I thought was happening.  Now I am only
reporting what actually happens.

John T. Fitch
E-mail: JTFitch at FitchFamily_com
Web Site: www.fitchfamily.com/aquarium.html

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