What do you want to filter out, and how big is your tank.
For phosphate removal, the reactor is designed to use 150g (or mL, the small container) of PhosBan.
I have a Kent Phosphate Reactor (basically the same as yours) that I am going to try Magnavore PURA Complete (carbon, phosphate remover, and selective resin). Word of warning though, a filter bag may be needed with the PURA (don't know yet), and that will limit the ability to fluidize the media.