Iain Miller wrote:
[snippety-snip]


KH can possibly matter if one *is* using CO2, though. Then, KH
determines [CO2] at a given pH

Kind of the wrong way round I think. Perhaps better put that at a
given
level of Kh then Ph varies depending on the level of CO2 in the tank.


I.


We know that KH, pH, and [CO2] are intimately related by this equation:
[CO2] = 3 * (KH) * 10 ^ (7-(pH))

I said that if KH and pH are constant, then [CO2] is the variable. In
other words, at given KH and pH, one can solve for [CO2]. That is
directly observable in the foregoing equation, and it is mathematically
sound. Again, we assume CO2 is being injected and won't equilibrate to
2-3ppm with the air.

You said that if KH is constant then pH must vary with CO2. That, too,
is directly observable in the foregoing equation.

Neither view is more correct; each is a mathematical truism.


Anyway, trying to rope this back into a discussion about plants/algae,
here's what I meant: Let us say we have 2 aquaria, A and B, and we set
the CO2-controller to maintain pH==6.5 in each (we heard "tetras like
pH 6.5" or something). And let's say we also want to grow plants (we
heard "plants like ~25ppm CO2 and below that algae win out.")

Here's what happens when we hold 2 of those parameters, KH and pH,
constant:

Tank A: KH==1.5, pH==6.5 [CO2]==14.2 ppm

Tank B: KH==3.0, pH==6.5 [CO2]==28.5 ppm

As you see, varying KH at a given pH DOES determine [CO2].

So in either tank the fish will get the pH they purportedly like. But
only in tank B, where the aquarist maintains a KH ~3, will there be the
proper [CO2] for plants.

Put another way, if you're using CO2 injection and are maintaining a
set pH, then the way you regulate CO2 concentration is by modulating
KH.

If you are not using CO2 injection, you're stuck with 2-3ppm CO2
(atmospheric equilibrium) and that's that; you dial in pH by modulating
KH. Assuming, also, that we're only using the bicarb/CO2 buffer
system. g

I hope that made it more clear.

--Trapper