I guess (getting way off topic here), that that's also the reason we use calcium hydroxide or calcium chloride to buffer our calcium levels, isn't it? Calcium carbonate, while giving us exactly what we want (a 2-part additive in one, say), dissolves so bloody poorly that it wouldn't get us very far.
Might also explain why mixing calcium carbonate, sodium bicarbonate, and water in the same vessel doesn't help either. From what I gather, combining your additives together lets you precipitate calcium carbonate, heats the whole mess up, and foams like hell. It was actually rather fun to watch.
Er, and that tank I've seen firsthand with a 5s pH? That was my first freshwater tank, before I'd heard of pH.
