It depends on how hard your water is, but if it's very hard and the RO system gets lots of use, you need a softener or anti-scalant injector that injects a smal

It depends on how hard your water is, but if it's very hard and the RO system gets lots of use, you need a softener or anti-scalant injector that injects a small amount of chemical which prevents scale buildup. If you don't do this and have particularly hard water the membranes foul pretty rapidly. In the case of undersink systems pumping out two gallons or so a day from standard municipal water, it's probably less important. I only know this because I've been playing with osmosis systems for a few years and have built one that does around 150 gallons a day from undersink system parts with 2 membranes, some filters and pumps for my garden (otherwise my water is simply too hard to grow all the things I want to grow).