CaribSea ARM Calcium Reactor Media, Coarse, 8 lb, is a complete reactor media containing not only calcium and carbonate, but essential trace elements as well. ARM has nearly 50 times the strontium of other brands (7,390 ppm), is processed for immediate use, does not require rinsing, and has double the surface area of regular aragonite. Precision grading (2-3mm) allows for maximum carbon dioxide penetration, and the exclusion of gastropod shells ensures the lowest phosphate (less than 1/10 of 1 ppm) and silica (about 1/2 that of other brands) content. This grading also carries in the highest concentration of Halimeda incrassata, an aragonite so pure it has been used as a laboratory standard. ARM has the highest solubility of any reactor media available (metastable at 8.2), so CO2 consumption is much lower. ARM is essentially what your corals are made of. 8 lb.