I have two somewhat large systems that use a lot of water. 200 total volume and 300 total volume. The tds on my well water is 400+ coming in and my R/O takes it down to 15, but that's still pretty dirty. My 2 DI's (one on each RO)gets changed out often and I've been neglecting that part of my routine because of the costs. I figured I'd buy a 50lbs of it and split it with some folks bringing my $50 down to around $20 for a 5lb bag.