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CO2 measurements (and BBA again)

>>>I do wonder though, if you could take
sample of your tank water and let it sit
open overnight, could it be assumed that
there was no more than 2 ppm CO2 in the
sample? Then, if the pH difference
between the water in the CO2 injected
tank and the sample taken the night
before is not more than 1, then can one
assume there is less than 20 ppm CO2 in
the tank? In fact in general, can it be
said that if the pH difference in this
kind of test is less than 1 then your
fish won't be in any danger of CO2

>>Would running an airstone at night and
taking a pH reading in the morning be
>a good starting point in the same way
as above? I would imagine this
>dissipates much or most of the CO2 as
well . . . ? So the calculation is not
>an absolute then, but relative? I was
trying to go by the book on this one .

>Notice that my post is all questions and
>no answers. I am hoping that someone can
>tell me. I wouldn't draw any conclusions
>at all from what I said. I am just
>looking for a rule of thumb for
>determining safe CO2 levels in tanks
>that may contain non-carbonate buffers.

Wayne, I just answered my own question. Last night I collected a water sample 
and left it sitting in a vial till morning. I ran an airwand all night and 
collected a sample this morning. While the pH last night before lights out 
was approximately 6.5, the vial which I collected and remained sitting was at 
6.8 this morning. The sample collected from the tank this morning which had 
been bubbling all night had risen to 7.0. Another point of interest, 
yesterday morning my pH was at 6.8 after running the air moderately in the 
tank all night (I can adjust with a valve). Last night I ran the air full 
force, and pH had risen a bit more.

Like you, I am trying to determine adequate CO2 levels, safe for the fish, 
and effective for the plants. The fish looked a bit peculiar to me yesterday. 
I don't know if this is because of the great fluctuation in pH, or lack of 
oxygen, or high CO2. The one largest sword could be seen with oxygen bubbles 
running through and off the tips of leaves. This is the only plant with this 
obvious indication of appreciation for CO2 (and it unfortunately has the 
greatest invasion of algae--beard, I think). My other plants are not doing 
substantially better, and in fact, some are not doing as well as before CO2 
addition. I actually wound up clipping off many of the leaves of this sword, 
as I've noticed that though the red brush algae is diminishing on the one 
smaller sword plant since starting CO2, the beard algae which is on my 
largest sword is growing. After this, the traumatized sword also stopped 
"pearling". So I'm wondering whether I should even continue with the CO2 . . .

In summary then, I guess the 7.0 would be my benchmark for the lowest level 
of CO2 in the water . . . I wouldn't allow the pH to drop by a factor of one, 
as that would be too great a fluctuation for the fish I would imagine. I'm 
thinking perhaps .4, .5 maximum over the course of the day? Have no idea what 
level CO2 that would provide . . . obviously those neat tables are not