Seachem Reef Kalkwasser 500 ml.

Reef Kalkwasser™ is a pure calcium hydroxide with unsurpassed purity and solubility characteristics.
Calcium hydroxide is used to prepare limewater (kalkwasser) solutions that will maintain calcium concentrations at natural seawater levels.
Kalkwasser solutions maintain calcium directly and carbonate alkalinity indirectly. However, its capacity to perform these functions is dependent on evaporation rate and CO2 concentration. If the these two factors are insufficient you can provide additional calcium with Reef Advantage Calcium™, Reef Complete™, or Reef Calcium™, and carbonate alkalinity with Reef Builder™ or Reef Carbonate™ (do not mix these directly with a kalkwasser solution).
While Reef Kalkwasser is a high purity and economical limewater mix, the beginner, or someone with not as much time to devote to their system, my prefer to use other less involved methods for maintaing proper ionic concentration levels. (see right)

Directions:
Never add this product directly to the tank. Add 3 g (1 teaspoon) to 4 L (1 gallon*) of water and mix to dissolve. If using pure water, a fully saturated solution is prepared by adding 6 g (2 teaspoons) to 4 L (1 gallon*). Drip this solution into tank. Monitor pH and carbonate alkalinity. Adjust rate of addition to maintain a stable pH (8.3–8.4).
HINTS: Adding 4 L (1 gallon*) of a saturated kalkwasser solution to 150 L (40 gallons*) will raise calcium by 24 mg/L. Always use the clear supernatant (top solution) only and do not allow undissolved material to enter the aquarium. Keep this container tightly closed when not in use and minimize solution/air contact to avoid calcium carbonate formation/precipitation.