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Re: [APD] KH question

> Date: Sun, 19 Nov 2006 10:20:44 -0800
> From: "Liz Wilhite"
> Subject: KH question
> ...The KH in the tank with aragonite is about 6, but the
> Eco-Complete tank is running about 3.  I am just getting
> my CO2 up and running again and I have a question:  with
> a KH of 3 do I run a risk of the pH crashing?  If I need
> to raise the KH, could I expect to see a consistent,
> stable KH level if I added a bit of crushed coral to the
> canister filter?

The questions and concerns of "pH Crash" have already been adequately
answered by Wright - the "crash" is more a concern of long- term tank
maintenance, nitrification and fish health than of acidic stability. The
fish stay much healthier if the mineral content of the water remains more
nearly constant.

As to the question of adding coral to the cannister, however...

1. CO2 does not "consume" a carbonate buffer in the manner of mineral
acids - it is, instead, an integral part of the carbonate family of buffers.
When CO2 is dissolved in water it adds (bi)carbonates along with the
hydrogen; conversely, both hydrogen and (bi)carbonates are removed when CO2
outgasses from the water. PH shifts here because we shift equilibrium ratios
among the three proportions more than their overall, total levels. No matter
how much extra CO2 you add to the water, the only thing that ever comes back
out is the over- pressurized CO2. Results are predictable and easier to
control as the system's "steady state" is more- or- less maintained.

Mineral acids or bases _do_ consume the buffers, replacing either the
hydrogen or the (bi)carbonates with one of their own components - like Cl-
or SO4-- with acids or Ca++, P+ or Na+ with bases. Here you're putting
something else in and outgassing CO2.

2. Increasing the proportion of H2CO3 in the water naturally makes the water
more acidic, and the dissolution of compounds it comes in _contact_ with is
made all the easier. In nature, it is more often the higher concentrations
of carbonic acid induced by high levels of CO2 (like rainwater seeping down
through the ground containing decaying organic matter, or underground water
sources enriched through volcanic outgassing) that allow, say, calcium
carbonate (a relatively hard- to- dissolve compound) to gain "easy access"
to the water table. In such a case, the acid will help more of the calcium
dissolve into the water, and in order to remain at a neutral charge balance
more of the (bi)carbonates will become "bound" to the water column. Thus,
pH, KH and GH tend to rise - particularly the GH as H+ is replaced by Ca++.
This is much more difficult to predict and control since the system's
"steady state" no longer remains so, and the degree of imbalance relies on
factors such as the amount of solvent, the total surface area of the solute
and the contact time between solute and solvent come into play.

With those facts in mind, let's look a little more closely at the situation
you present:

First off, eliminate "old tank syndrome" (nitrification) and its resultant
"pH crash" by assuming either frequent water changes (because we all know
you to be the type who actually "maintains" her tanks), the consumption of
nitrogen by the abundance of plants, or a simplified combination of both.
This also enjoys the beneficial elimination of all the tangential arguments
and distractors.

Adding CO2 for the benefit of the plants will not affect the KH in and of
itself. All things being equal, this state of affairs lets you worry about
nothing more than the level of CO2 in the water column.

Adding calcium carbonate _will_ affect the KH, but by how much? Now we have
to look at:
     1. How much CaCO3 is present in the shells I'm using? Will this
composition remain fixed from one handful of shells to the next?
     2. How much surface area do the shells present? Are they large pieces,
with a lower surface area- to- volume ratio, or are they in finer pieces
with a disproportionately higher SA:V ratio than expected. Should I sift
them for a consistent size? How fast will they dissolve?
     3. How fast will my KH start shifting? Will there be more of a shift at
night, when the CO2 concentration is higher, or should the CO2 be turned off
at night to sort of "balance" the dissolution rate? Can I just take a daily
average or a weekly one?
     4. Since it's hard to predict just how much or how fast the KH will
shift, how do I know how much CO2 I'm adding without constantly testing just
about everything in the tank? Will I get into a "Catch 22" situation by
having to constantly adjust the CO2, knowing that this adjustment will, in
turn, once again change the dissolution rate of the calcium carbonate?
     5. ...Well, you get the point...

As they used to say all throughout my old calculus texts - by now, the
answer should be "intuitively obvious to the astute observer". And with that
I leave you to draw your own conclusions...


David A. Youngker
jaafaman at comcast_net

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