Aquarium Systems Test Kit A new range of test kits from Aquarium Systems that accurately test your marine and freshwater aquarium water. The Seatest Magnesium (Mg) test kit from Aquarium Systems contains approximately 40 individual tests, allowing you to accurately measure levels of the third most important element is the composition of sea water. Magnesium is important for both photosynthesis and construction and formation of the skeletons of reef organisms - ensuring levels are correct is essential for the health of reef aquaria. Please note this product is short dated - BBE August 2015.