KH and pH

["Jennifer Marshall" <zephyr18 at ev1_net>]

I slept through my high school chemistry class, so please bear with me:
I have a 20gal plant/community tank with no added CO2.  Even with weekly water changes, I was getting chronically low pH (6.2-6.6) and a KH of 1 if any. I started adding a shmidgen (very scientific measurement) of baking soda to each gallon of fresh water during water changes. This definitely raised the pH; I'm now at a more comfortable level of 7.0-7.2, but my KH is still a low 1-2 and continuously drops between the baking soda shmidgens  The pH of my tap water is of course way off my chart, which ends at 7.6, and the KH of my tap water is 3-4.  Just in case it's all related: the GH of my tank is about 7-8 and that of my tap water is about  9-10.
SO, my question is: what is it that makes the KH not stay constant? Would the two very very small pieces of driftwood affect the KH as it supposedly lowers the GH? Is there a more efficient way to keep the KH, and thus the pH, constant? Does KH always fluctuate and I am just being neurotic? 
I just got a 68gal tank and I am doing all kinds of research on using carbon dioxide, but before I start using any, I want to make sure that I have solved my KH/pH mystery.  Any advice?

Thanks, 
Jen


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