Kalkwasser is Calcium Hydroxide, Limestone is Calcium Carbonate similar to Aragonite (the stuff recommended for substrate) which has a slightly different crystal structure that makes it easier to dissolve at pH's usually found in aquariums (7.8? I think?)
Anyways... Limestone = "Dead" Base Rock
Lime (Kalwasser) = Happy Corals
but if you're not careful, excessive pH
and perhaps the dreaded "calcium precipitate snowstorm"
I'll shut up now.