When Calcium Hydroxide solution (Kalkwasser) is slowly dripped into your aquarium, it captures free Carbon Dioxide present in the tank water and converts it to Bicarbonate ions (which is a good thing). The use of kalkwasser (lime water or calcium hydroxide) is used to maintain calcium, alkalinity, and pH in reef aquaria. If done properly it can be very effective at having a beautiful thriving reef show aquarium. An alternative would be to use a Calcium Reactor with a CO2 aquarium system.