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Re: Kh, Ph, Buffering, and Baking Soda.

> So, I want to ask anyone who knows... Previous to doing this test, I had
> understood that the term Ph Buffering refers to using a substance that adds
> alkalinity, (the ability to absorb acids), without actually changing Ph.
> But in this test, the Ph changed along with the Kh.

I'm way out of my league (based on the the Nitrogen/photosynthesis discussion
thats going on!), but I'll try this one:

pH is a relationship between your buffering and any acids present.  
In a really REALLY crude way, pH = buffering / acids.  (way over-simplified,
I know).

If you increase your buffering, your pH will rise.
If you decrease your buffering, your pH will fall.

If you increase the acids (such as by adding CO2) your pH will fall.
If you decrease the acids (reducing CO2 levels) your pH will rise.

> So, if I added CO2 to the water described above, would there be a
> precipitous, unmanageable, Ph drop?

Something is not right in your listed parameters:

With a KH of 4, your pH is 6.5?  Either your tank isn't buffered
with carbonate, or you've got some other source of acid lowering
your pH.    With a KH of 4, without added CO2, the normal pH would
be around 7.6 - 7.8.   So something isn't right...

I would not recommend adding CO2 to those conditions.   I'd try to 
find out what else is altering your pH first. 

Do you use any pH adjusting additives?