Calcium is one of the most important elements in a reef aquarium. Reef invertebrates, such as corals, crustaceans, mollusks, and even some type of encrusting algae, need calcium to grow and remain healthy. Over time, the calcium level drops as it is used by invertebrates, or as it reacts with excess phosphate. The API CALCIUM TEST KIT measures calcium levels as low as 20 mg/L to help you keep your reef inhabitants healthy.