A calcium reactor uses CO2 injected into the water to decrease the pH in the chamber low enough to dissolve the media, thereby releasing Ca, alkalinity, and entrapped Mg. Saltwater from the system is then cycled through the reactor, picking up the dissolved chemicals and transporting them into the system.
By contrast, Kalkwasser is created by adding calcium hydroxide powder to RODI water and has a very high pH. It is typically added as either a drip or as part or all of the top off water.
If your calcium demands are low enough, Kalk delivered via your ATO can be enough to maintain levels in the aquarium. Once the demand exceeds what the ATO can deliver, a Calcium reactor is used as it is independent of the evaporation rate. Using both is typically done to balance the pH in the system closer to the ideal.
