Watts Flowmatic W-1812-50 RO Membrane is a 50 gallon per day reverse osmosis thin film composite (TFC) membrane. Fits most standard RO units including but not limited to the Flowmatic FMRO4J FMRO4G FMRO5J FMRO5G FMRO5GP FMRO4ERP FMRO5-C FMRO4-ZW F