Changing KH with KOH or Ca(OH)2

[eric at alchemy_cchem.berkeley.edu (Eric Tulsky)]

OK. My Lamotte CO2 and KH test kits finally showed up yesterday, so I
went a-testing. CO2 is at 55 ppm, KH is at 10 ppm, which gives pH of
about 5.4 (not completely inconsistent with my measurements).

So I figure the KH needs to go up and the CO2 should probably come
down. And I think it's likely that Ca levels are low if KH is that
low, right? (Is this a safe assumption?) So I went looking through
the archives for the best way to do that, and found that people seem
to recommend using CaCO3 to raise both KH and GH, using NaHCO3 to
raise just KH, and Ca(OH)2 to raise GH but not KH. It's this last
one that caught my attention.

Ca(OH)2 will release free OH- ions. These will rapidly react with
any acid they find -- in the aquarium solution, that'll be the CO2
(which is mostly present as H2CO3). The reaction will be:

     H2CO3  +  OH-    ----->     H2O  +  HCO3-

So now you've just lost one molecule of CO2 and gained one ion of
HCO3-, which is part of the KH, right? So it seems to me like adding
Ca(OH)2 is going to raise GH *and* KH, at the expense of your CO2
levels. Am I missing something here?

Interestingly, if this is right, then you can use NaOH or KOH or
NH4OH or any other base that strikes your fancy to raise KH without
raising GH (again, at the expense of CO2 levels).

Any thoughts? Have I overlooked something?

Eric

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