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CO2 x Kh

Im intending to add CO2 to my tank, which has the following measurements:
Ph - 6.8
Kh - 3.0.
Considering those measurements, I think that adding CO2 at this moment would
produce a sudden drop in Ph, which is not a good thing.
So, Im thinking in, first, raising Kh (by adding some sea shells to the
external filter) to a level of, say 5 or 6 degrees, and then adding the CO2.

Now, I raise some questions:
1 - Is the above strategy correct?
2 - Once the Kh desired level is achieved, will it be stable (considering
that I will also correct the Kh level of partial change water) so I can pick
out the sea shells, or must I maintain the sea shells forever in the filter?
3 - Is it possible to raise the Kh without adding more CO2? This question
raised in my mind after reading the following (extracted from a discussion
in th Krib):
"The *solubility* of calcium carbonate [CaCO3] in *pure* water is only
about 17 ppm (i.e. about 1 "German degree"). It is a well known fact,
that calcium BIcarbonate [Ca(HCO3)2] is much more soluble in water than
calcium carbonate. Calcium bicarbonate is unstable, exists only in
solution, one can not isolate it & put in a bottle. The way one prepares
it, is to add CO2 to the water which is in contact with excess solid
CaCO3. The CO2 (actually carbonic acid, H2CO3) reacts with the calcium
carbonate in solution, forming the bicarbonate, and shifts the
equilibrium, allowing more solid calcium carbonate to dissolve... and so
If enough CO2 is present, a lot of CaCO3 can dissolve."

Gustavo Adolfo Lapido Loureiro
(21) 2507 - 5464

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