Limestone is calcium carbonate. If you heat it up hot enough it will decompose into calcium oxide (lime), which will absorb water from the atmosphere to make calcium hydroxide (slaked lime). After years and years, the calcium hydroxide will reabsorb carbon dioxide to reform calcium carbonate (limestone).
I was thinking that you might be able to put a single big block of limestone in the tank that would very slowly dissolve and reduce the number of calcium carbonate additions you needed to make.
@makinstuff just found out what happens if you put way too much calcium carbonate in the system. His aircrete was dissolving too quickly and pushing up the pH too much.
Limestone is not as porous as fresh aircrete, so it will dissolve more slowly. It would also be pretty easy to just pull it out of the tank if it keeps the pH too high.
RE: Aquaponics Tips #2