RO question Vancouver peeps

[jostafew]

I've never changed any of the 4 stages of my RO DI in the past 3 years or so that I've been reefing. I measure 15-20ppm coming out of the tap (pretty good!) and have never measured anything but 0ppm out the other end of the filter so I've never had a reason to change. FYI I've got a 75gpd RO/DI supplying water for a system with total volume of approx. 100gal. I make on average about 20gal / month in makeup water plus a few extra gallons here and there for WC.