In tandem with the HI783 Marine Magnesium Checker HC, the HI783-25 Marine Magnesium Checker HC Reagents are pre-made high-quality reagents for ease of use while checking Magnesium levels in your aquarium. This reagent allows you to accurately measure high levels of Magnesium (1000-1800 ppm) in saltwater and marine biology applications. Refill provides enough reagent to conduct 25 water tests.