Calcium testing and addition Calcium is a major constituent of calcareous algae, skeletal material of hard corals and the skeletal needles of soft/leather corals. Calcium also fulfills many biological functions. Quite often a too low calcium concentration retards coral and calcareous algae growth. NSW contains 400 – 450 mg/L calcium. Which is also the value we should strive for. For a stable aquarium environment the fluctuation in calcium content should preferably not more than approx. 15 mg/L. This automatically means that the calcium test should be able to monitor comparable fluctuations. Conclusion: In a healthy aquarium growth and multiplication of corals, desired calcareous algae and other organisms results in calcium depletion. Because of the importance of calcium timely corrective measures would have to be taken. For that purpose a proper calcium additive together with an accurate test kit are required.