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Re: pH and the uptake of CO2

Aquatic-Plants at actwin_com,EOL-Inet writes:

>Roger Miller

>There might be a little misconception here.
>The starting pH of an aquarium doesn't limit the CO2 concentration that
>you can get in a tank.  You can get 15 ppm CO2 into a tank starting from
>pH 10 almost as readily as you can get it into a tank starting from pH 7. 

I can't get 15ppm CO2 in my tank when it's got a pH of 8.2.  Not according
to the ph vs KH tables anyhow.

>  CO2 concentrations
>are controlled mostly by the amounts of CO2 moving into and out of the
>aquarium and the amounts being used up or generated in the aquarium.
> Normally,
>though the alkalinity isn't that high and a pH of 8.4 implies a pretty 
>low CO2 concentration. 

These last two paragraphs seem to contradict each other.  If CO2
concentration is controlled by the amounts of CO2 moving into and around
my aquarium, then why does my CO2 concentration go up when I lower the pH,
when my CO2 injection rate stays the same?
>At low CO2 concentrations and with other things being conducive, some
>plants will start using bicarbonate as their carbon source, and some will
>not.  I don't think there is any pH where that change occurs, and
>certainly not one that's applicable to all plants. 
At a pH of 8.2 my plants do not bubble and generally die back.  When I
lower the pH (and carbonate hardness as a result of the acids in the water
conditioner) the plants pick up and stream O2.

Thus, I can only conclude that the CO2 is in my aquarium all along, but in
some for that the plants cannot make use of.

Any replies, comments or general blurb to Steven Amor:
Steve_Amor at eol_ieaust.org.au

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