Magnesium and calcium concentrations and the ratio between these two elements are constantly changing in marine aquariums due to assimilation into the skeletons of corals and coralline algae and the process of precipitation, A periodic check of magnesium and calcium concentrations is necessary to determine the amounts of these elements that needs to be added to the aquarium in order to restore them to natural seawater levels, The Magnesium-Calcium-Combitest consistently determines the exact concentration of each of these elements separately, Each Magnesium-Calcium-Combitest kit contains reagents for approx, 30 Tests, SKU: RYLHC6324