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. For all membranes, RO output depends on the source water temperature. At 77, the system should produce 100% of the rated output. As the temperature drops or increases, so does the output