BioMaster prefilter foam sets come in 4 or 6 pack.
BioMaster 250 - 4 foam - 4set
BioMaster 300 - 5 foam - 6set
BioMaster 600 - 6 foam - 6 set
BioMaster 850 - 6 foam - 6 set
from factory the foam is 45ppi (blue) if you have lager fish this will not get blocked up as quick.