A Kalk reactor is different than a Calcium Reactor. A Kalk reactor used Kalk powder which is mixed in a freshwater solution. The freshwater solution is then dripped into your sump usually during the lights out hours of the tank to raise pH or with your auto topoff system.
A Calcuim reactor uses basically old coral skeletons and dissolves them in a mixture of CO2 and salt water from your aquarium. The CO2 lowers pH to about 6.5-7.
Kalk seems to raise Alkalinity and maintains it quite well but not so much for Calcium levels. The solution is very basic (i.e. pH 12 or so) so care must be taken when dosing Kalk solution.
Calcium reactors raise Calcium levels and maintains them well but do not maintain Alkalinity as well as Kalk. The solution has a moderately low pH (usually 6.5-7).
As for building either, I would say a Kalk reactor is *slightly* easier to build than a Calcium reactor since it has fewer parts, but neither of them are terribly difficult to figure out and build. The inital cost of setting up a Calcium reactor is higher, but the overall maintenence of it is lower than a Kalk reactor.
__________________
32"x32"x20" Cube-ish tank
|