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Re: Changing KH with KOH or Ca(OH)2

On Tue, 28 Sep 1999, Eric Tulsky wrote:


> 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?)

This might be fairly safe, but be careful not to turn it around; it isn't
safe to assume that high KH implies high GH.  There is often some
relationship between GH and KH but there are many, many exceptions.  GH
test kits are pretty inexpensive so it's usually best to just get a GH

> 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?

There's also:

	H+ + OH- <-> H2O

and a few other things going on at the same time.  Enough so that it might
be difficult to predict exactly what change in alkalinity (KH) you might
get from adding the Ca(OH)2, but there will be some increase in

If you want to raise GH without raising alkalinity you can use CaCl2,
MgCl2, or MgSO4.nH2O (to name a few).

> 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).

Yep.  You can do that.  The CO2 level rebounds after the reaction is

Roger Miller