As a bit to add:
I was reading through Fishnets library and came across this from the Advanced Aquarist forum;
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The level of magnesium will determine if your solution will form aragonite or calcite. Corals and shellfish generally only form aragonite. If you are looking for maximum biological calcification you must not have excess magnesium.
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As well Andrew brought this to my attention;
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Mg++ allows Ca++ to go up but if it goes up beyond a reasonable ratio it will cause alkalinity to drop -- you know the whole Ca++ Alk balance thing.
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I looked it up and found reference to it on Fishnet during a discussion with Randy Holmes Farley about dosing Epson Salts to boost low Mg++ levels:
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Bob –
Given that Magnesium bonds tends to form on the xtals(crystals) of Calcium carbonate
thus inhibiting their dissolution, is it not possible that the reduction of
the total #s of CO3= in solution might decrease the pH?
Randy Holmes-Farley - Whether it did that on it own as MgCO3 precipitation (or something else like
Mg(OH)2), or on the surface of CaCO3, that is how I believe the pH would drop.
Of course, in the test I ran there was no solid surface of CaCO3, but in a
tank there would be.
Buffering effect would not be changed for any period as the SO4= would stand
in until the CO2 infusion would re balance.>>
I don't agree with that. The buffering (alkalinity) will be depleted. This
would be true whether you believe that hydroxide or carbonate or both were
removed from the system. Even after reequilibration with CO2, the solution is
permanently decreased in alkalinity, and will reequilibrate at a lower pH.
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Just something else to consider.