The hel-x 13 biomedia for the biomaster 250, 350, or 600 includes a replacement biomedia allotment. The allotment contains 0.3 gallons of hel-x biomedia. Hel-x biomedia has a large surface area across this Suspending media form which is ideal for biological filtration. It is best to only replace the biomedia when you do a complete tank restart as to not disturb the beneficial biological load.