Replace membranes when product water decreases and/or ppm increase. Merlin Garden Pro and Evolution RO1000 membranes reduce 94%+ of total dissolved solids. Stealth RO100/200, Stealth RO150/300 and microRO-75 membranes reduces 98%+ of total dissolved solids. The Evolution RO1000 and Merlin Garden Pro systems need two (2) membranes.