Testing Alan's tank....dam you lumpers!

[Delphinus]

Yes, that is correct. You cannot raise one too much without dropping down the other. I think of Alk as a measure of "the ability of calcium to precipitate." The higher it is, the more likely Ca will precipitate out, thus the tested value of Ca in the water column will drop.

Any of the a/b additives, tell you you cannot add them to the tank at the same time. One should be morning, the other evening. I use a little bucket to predissolve the stuff, one time I forget to completly rinse out my container between additions, so there was a little Ca residue when I mixed up the buffer. The reaction it made was pretty cool! Well, hot, actually, because it's definitely an exothermic reaction (the container felt like it was containing boiling water for a second!) It took me forever to chip out the calcium precipitate out of my little bucket.

I wouldn't use baking soda on its own for buffer. I forget the exact reasoning, but it's not complete, over long-term it will lead to ionic imbalance or something like that. What you need to do, is mix up 5 parts baking soda to one part washing soda (or something like that. You should be able to find the exact recipe on RC ... many people are doing this).

The reason people like the two part (or the a/b as you and others sometimes call it) is that you can zero in on one value and adjust that one independently, leaving the other one alone. Kalk and even Ca/Rx, you can't do it independently.