http://www.bulkreefsupply.com/store/products/carbon-and-gfo/bulk-gfo-granular-ferric-oxide-phosphate-remover The purpose of GFO is to inhibit algae growth by removing phosphates from the aquarium. Maintaining low phosphate levels will help prevent algae outbreaks and control growth. Serious algae outbreak is one of the most common reasons for complete tank shutdown. GFO is an essential tool and should be part of regular tank maintenance.