GFO - Grandular Feric Oxide - Do I Need It?
High phosphate levels are a recipe for disaster in saltwater aquariums. It is a nutrient for nuisance algae and also hampers the formation of calcium skeleton in corals. There are several different methods for dealing with phosphate levels but one they I like is using granulated ferric oxide (or GFO) in a media reactor. GFO is best used with a media reactor for several reasons.
First, it is a very fine media and would easily escape from any bag you put it in.
Second, it clumps together if you don't have water moving through it.