Calcium is a major constituent of coralline algae, the skeletal material of hard corals and the skeletal needles of soft/leather corals. In many cases too low a calcium concentration retards coral and coralline algae growth. \u2022 Calcium also fulfils many important biological functions. \u2022 In a healthy aquarium, the growth and multiplication of corals, calcareous algae and other organisms can result in calcium depletion. To maintain a stable environment it is important that the calcium content should not fluctuate by more than 15 mg/L from the optimum range of 420 - 450 mg/L so regular testing should be carried out. \u2022 This means that any calcium test used should have an accuracy of significantly less than 15mg/L otherwise it will be difficult to monitor such fluctuations. \u2022 The Salifert test kit measures in steps of 5 mg/L and has an accuracy of 10 mg/L. \u2022 The colour change is precise, detecting small but significant fluctuations in the concentration of calcium. 2