Re: CO2

Paul Sears says:

>>Consider CO2.
>>        Another nutrient which may be related to the sustenance of red
algae is
>>inorganic carbon. This exists in the aquarium as dissolved CO2, bicarbonate
>>or carbonates. The equilibrium of these carbon species depend on pH.  Free
>>CO2 becomes available at pH less than 8.0 and predominates when pH is less
>>than 6.5. 
>	The CO2 concentration in the water will depend on the interaction
>of the water with the atmosphere.  If you have no CO2 injection system,
>it will be only a few ppm, and if you inject CO2, it may be as high as twenty
>or thirty ppm.  The pH of the solution is irrelevant to this.  
>	If the pH is high, it just means that there is a lot of HCO3-
>in the solution _as_ _well_.  There will be little CO3-- at any pH of
>interest to us (below 8.5, say).

Why does the concentration only depend on the interaction with the
atmosphere? What if there is another source of CO2... like decomposition of
organic matter (including fish food). Isn't this the source of C in natural
systems where the CO2 concentration is higher than that predicted by the
atmospheric equilibrium levels. I have tanks with organic substrates that
run for years without CO2 injection. Secondly, once this CO2 is available in
the water, doesn't it enter into equilibrium with HCO3... and the percent
CO2/HCO3/CO3 is a funtion of pH. I had understood that the pH can be a
funtion of many factors, only one of which is based on the CO2 entering the
system. If the pH is lowered (from other factors), doesn't the HCO3 change
to CO2?

My knowledge of this process is based on the one or two paragraphs that can
be found in limnology texts. Please help fill in more details.

Neil Frank      TAG editor         Raleigh, NC