@Jenny: The original entry is definitely correct and should be kept.
You CANNOT translate Kalk with calcium. This would be incorrect. Confusion arises from the everyday use of chemical terms. Your english examples "calcium in the water" and "calcium build up" describes two different things, soluble calciums salts and the build up of CaCO3, which is insoluble. To call it "calcium buildup" is not really correct (since it is not calcium, which is a very reactive metal, but calcium carbonate), but it might be used this way in everyday speech. However, German layman speech makes the correct distinction:
"There is too much calcium in our water," = "zu hoher Kalziumgehalt im Wasser" or BETTER "zu hartes Wasser". You cannot say "zu viel Kalk im Wasser", since Kalk is the insoluble precipitate. (You might hear it said from time to time in German, but people do mistakes.)
"calcification", "calcium build up" (I accept your word for the latter one) = "Verkalkung", "Kalkablagerung"
I am simplifying the chemistry here, neglecting things such as Mg, HCO3- content, etc, so don't get on my back on that one. My main point is, that just because people are using (technically incorrect) "calcium" where they should say "calcium carbonate" or "lime", it does not invalidate the correct translation provided in LEO. I would support your additional suggestions ONLY as examples for layman speech. The one to one translation calcium=Kalk is just wrong.