Synthetic Ferric hydroxide material from Two Little Fishies. It has one of the largest absorption capacity when compared other similar products. Excessive phosphate levels can inhibit coral growth and encourages growth of algae instead. Therefore effectively removing phosphate is vital to the health of your corals and your tank in general. You can easily put them in a filter media bag and throw it in the sump where there is adequate flow or for best results, put them inside a PhosBan Reactor.