Rena Nexx foam filters are 30ppi foam filters that have the ideal thickness and pore size to trap small particles before they pass through the filter system. These filter pads can be used to replace the standard 4-sided API cartridges used in the Nexx filter. These filters will last for up to a year with just simple rinsing during maintenance. With these foams in place almost any filter material can be used in the filter canisters of the Nexx Filter so you can customize the filter in hundreds of configurations.?