And I'll add that the best way to treat staining is to remove the cause of the stains, not to treat the symptoms; the resulting stains.
Regular softeners can remove up to 1-2 ppm (mg/l) of ferrous, clear water, iron. And if you buy right, others will remove up to 5 ppm of iron.
Although a softener should not be used as a filter, although resin is a fairly good filter, only a few brands of softeners, actually the control valves used, require a prefilter. Most prefilters will cause more problems than they cure. They ca nactually cause the softener to be unable to remove all the iron in the water. If the water is clear when drawn, no softener requires a prefilter; IOWs it's more marketing than factual if someone suggests a prefilter simply because you are buying a softener.
Any softener that is allowing rust stains on fixtures needs a resin cleaner periodically and/or it is not sized correctly and/or is missing certain internal features to assure removal of the iron found in the raw water.
Gary Quality Water Associates