I have the same filter for my setup, i replaced all 3 of the bottom filters,1,2,3,, havent replaced any of the others yet, the manuel says the membrane will last 2-3 yrs, the 1,2,3,5th stage will last 6-12 months. i have a tds meter and have a reading of .01, I bought all of my filters from Filterdirect-water.com , they are preety cheep there.