Why?
kalkwassser is calcium hydroxide, or calcium oxide that hydrates and forms calcium hydroxide when put into solution. It has a very high non-carbonate alkalinity as there is calcium associated with the hydroxyl (OH-) anion.
When this is introduced to seawater, there is a very high presence of saturated dissolved carbon dioxide (pCO2) in the water column that almost immediately forms a
hydrocarbonate ion, then dissociates to form the bicarbonate ion. Fortunately for us in an equimolar concentration to the calcium ion in solution. This solution will come to equilibrium over an hour or so reaching a pH between 8.2 and 8.3 depending on several variables of the seawater and the atmospheric conditions. This solution forms both the calcium and alkalinity we need for corals in captivity by using kalkwasser as a topoff for evaporative losses from aquaria.
It is the perfect topoff solution, insoluble CaCO3 that forms precipitates out of solution, the good stuff forms a layer above the botom, and in most systems, the amount supplied by evaorative replacement will supply more than enough Ca++ and carbonate/bicarbonate alkalinity to keep moderately dense populations of
stony corals happy.
See
http://thereeftank.com/forums/showthread.php?t=62714 and
http://thereeftank.com/forums/showthread.php?t=64235 for more of the chemistry.
HTH
Bill 
