CO2 thoughts ...

I was discussing CO2 with Olga yesterday and thought some of the stuff
might be of interest to others.

[Olga wanted to increase the KH in her tanks]

> Well, my reasoning was to prevent pH swings. If I test in the morning
> the pH is about 6.5 - 6.6 when I test at night it's about 7. I assume
> this is because the plants don't use the CO2 at night -- but hardening
> up the water by adding more baking soda or some calcium carbonate
> would hold it steadier -- no?


> so my pH shouldn't rise due to
> adding hardness and would stop falling -- no?


Let's see ...

You have 1.8 dKH and the pH changes from 6.5 to 7.  That means the CO2
concentration goes from 17 ppm overnight to 5.5 ppm in the late
evening (those plants really suck up the CO2, eh?).  Note in this case
that when the plants are trying to photosynthesize the most (the late
afternoon), their source of CO2 is at a minimum.  Bummer.  This is why
a $$$ controller is good; it can provide all the CO2 the plants need
by increasing the amount delivered when it is neded.

[I've never thought about it this way before]

If you increased the KH to 3 dKH and kept the CO2 the same (with
yeast, you probably would have a hard time increasing it), you would
have a pH of 6.72 in the morning and 7.2 in the evening.  Hmmm, not
much difference, still a 0.48 pH change.  You've shifted the
average pH up a bit but only slightly changed the difference. 

If you increased the CO2 a bit to get the evening pH back down to 7.0,
you would need 9 ppm instead of 5.5.  Assuming the plants used the
same amount (perhaps incorrect), you would then have 20.5 ppm in the
morning, for a pH of 6.65.  Now the swing is 0.35.  A bit less and the
pH range is more neutral (is "neutral" better or not, who knows?).   

So, yes, it is more stable, but not dramatically.  And a lot depends
on how well you can control the CO2 flow.