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Re: How to maintain high PH/KH with CO2 injection

>Date: Thu, 20 Jan 2000 20:40:33 +0800
>From: Roy Cheok <roycheok at cyberway_com.sg>

>The problem is that the PH/KH needs to be relatively high...
>compared with the usual acidic waters of the planted aquaria.
>The question is....is it possible to maintain a high PH of 8 - 8.5, dKH
>of >8...in a CO2 injected tank that would be optimal (maybe not quite 
>optimal...but acceptable) for reasonable plant growth.

Not really. Even with a KH of 10, the "bare minimum" CO2 level of 10 mg/l 
will still give you a pH of about 7.5.  You would be better off looking for
plants that don't require high CO2 levels and settle for slow growing
"decorations" rather than a "real" plant tank.  Anubias are a good choice. 

BTW, do the fish *really* require a high pH or can they deal with any 
pH as long as the KH (alkalinity) is high?  Typically, the pH is not a
factor but just happens to be high in their environment due to the high
alkalinity.  For instance, discus do well at any pH as long as the GH
(calcium hardness) is low. The question may be hard to answer, since high
alkalinity usually creates high pH and perhaps no one has tried a lower pH
with the fish. 

>Obviously the injected CO2 will tend to lower the PH and KH...etc,

Please do further reading on the pH/KH/CO2 relationship. CO2 has nothing
to do with lowering or raising KH. This seems to be a common misconception.

KH and CO2 are both independent variables - you, the aquarist, set the KH
(by adding sodium bicarbonate or calcium carbonate, for example) and set
the CO2 level with the injector. The amount KH and CO2 then determine the
pH. Changing the amount of CO2 changes the pH only, not the KH. Changing
the amount of KH changes the pH only, not the amount of CO2. If you try to
change the pH by adding some of the pH adjusting products, you are messing
with the alkalinity such that the pH/KH/CO2 table is no longer valid and
you don't really know what you have.   

In practice, one usually decides how much CO2 is desired (for example, 15
and what pH is desired (for example, 7.0). From a pH/KH/CO2 table or chart,
then determines that a KH of 5 degrees will give a pH of 7.0 with 15 mg/l of 

The aquarist then adds enough carbonate of some form to achieve 5 dKH. This 
causes the pH to rise to some level above what is desired. Then the aquarist 
adjusts the CO2 injector until the pH comes down to 7.0. [Note that the
aquarist does NOT simply set the injector to "n" bubbles per second except
perhaps as a starting point]. The goal is to arrive at the desired pH,
giving the desired CO2 with the selected KH. 

George Booth, Ft. Collins, Colorado (booth at frii_com)