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Re: CO2 Level

John T. Fitch wrote:

>Since installing a pH probe and controller, I've been pleased with the way
>it holds the pH steady at 6.6, which, according to the chart, when
>combined with a KH of 4, provides me with a CO2 level of 30 ppm.  Ideal, I

>But this new instrument also reveals that during the night, when the
>lights are off, the pH usually drops to 6.5 or even 6.4.  When I look up a
>value of 6.4 in the chart, it says I then have 47 ppm of CO2.  My
>understanding was that level is too high for the fish.  On the other hand,
>I haven't noticed any unusual behavior, much less die off, on their part.
>So, my questions are: Should I be concerned and, if so, what do you
>recommend?  Thanks.
>John T. Fitch

The short answer is don't be concerned.  The recommended pH level for fish
is much more important to watch than the Co2 concentration.  The vast
majority of aquarium fish can easily tolerate a 6.4 pH (though a few
species may struggle).  Let your fish tell you what's good for them by
their behavior (gulping for air at the surface is the warning sign).  Keep
in mind a Co2 level is independent of the concentration of dissolved O2.  

Aquarium Plants & Driftwood