cheap tank buffering

>I wonder if baking soda can be used to buffer my soft water to prevent
>sudden decreases in PH. If so, what is the rule of the thumb to adding it to
>water? I am currently using Dupla's KH increase tablets. They're good, but
>way too expensive. Any replys are appreciated.

I use both baking soda and saltwater buffer (Seachem Marine Buffer).
I'm trying to raise the KH/GH of my water, as the pH is reasonable and
the water here at work has a KH/GH of .5/.5 (honest!).

So far, the baking soda seems to be raising the KH (I'm up to 2, from
0), but I haven't noticed a significant change in GH.

I add very small amounts when I do water changes every other week.
Currently I'm adding a couple of teaspoons of each to the water I put
back in.  This is on a ~45g octogonal tank with some plants, four, otos, and
four small cats (two glass and two cory).