The ones we use have medium that's pretty easy to change, and you have to, about yearly, as they say. The big expense in our standard ro/di's is up front, when you buy the rig. The medium is cheap, after that---open a cylinder, dump in some stuff, close the cylinder---which reminds me I need to do that this week on two of my cylinders. I have a perhaps unjustified suspicion they will make their money supplying you with filters of their size, and because they're opaque, you can't see when they're worn out. (color change, among other signs). If they don't produce 0 TDS water (no dissolved solids) they're worthless. If they do, that's what you need. Only a TDS meter can tell you that. And the question is, how long, and do you have 65 pounds water pressure in your house---and are you going to have to call a plumber to attach the thing?
