I have had good luck with the refillable DI cartridges from http://www.buckeyefieldsupply.com. I like the prices and the DI resin I get from them. I have been using a .5 micron sediment filter (dont see them on the site anymore), a .6 micron Chlorine Guzzler carbon block filter, and the refillable nuclear grade resin. So far, so good. I have about 232ppm from the tap, about 5ppm after the RO membrane, and 0ppm after the DI. The better the water quality going into the DI the longer it will last. This is from a 50GPD unit.