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pH-KH-CO2 Table

> I am using the pH/KH table at
> http://www.thekrib.com/Plants/CO2/kh-ph-co2-chart.html to determine the
> level of CO2 in my tank. I notice, though, that it says "The CO2-KH-pH
> equilibrium should be correct
> unless you are taking extreme measures to adjust pH, such as adding
> lots of phosphate buffers like pH-UP or pH-DOWN or using sulfuric
> acid." Now I do add some phosphate to my tank - maybe a BB-sized ball of
> KH2PO4 weekly. Will this change my readings?
> > - -Rachel

In a word yes.  The table assumes that the only buffer system in the water
is the carbonate buffer system, and adding another buffer will throw off the
calculations.  I hadn't seen the "extreme measures" clause, but I wish Eric
hadn't put that in there.  In my experience, *every* single time the table
is checked against readings from a higher end CO2 kit, such as the LaMotte
kit, the table comes out too high, sometimes ridiculously so.  If that table
were accurate, either my fish should be dead, or they're all some mutant
strain that can handle CO2 concentrations of up to 400 ppm with only a DIY
bubbler. Somehow I don't think so. Plants produce all sorts of changes to
water chemistry which I'd be willing to wager also throw off the table.
Now, one very important caveat is that my tanks are often below ph6, where
the table is very, very sensitive to small errors in pH measurement.
perhaps the table's more useful around or above a pH of 7, but even then,
you'd really need to have a good pH measure.

IMHE, the table's a good tool to get people to think about measuring CO2,
but it doesn't provide a useful function except as a kind of teaching tool.
This should of course be read as a call for testimonials of the fantastic
value of the table!

At any rate, it sounds like things are growing well, and looking good, which
in the final analysis is what matters.  I'm glad to hear the tank is doing
well (and you too!).