I got this system as a back up for a past home we owned, now its in the way. The water filters through the gravel and oxidizes the rust and other contaminates. Then it backwashes the contaminates out every few days. No salt needed. Sells for about $700 new.
I took some screen shots of similar, new units with the description of what this does. The tank I have is new, with new gravel. The control head is a refurbished unit.