For a prefilter, you might just use a small housing
(
http://www.waterfilters.net/culligan/wholehouse/housings/HF360.htm)
and a basic 5 micron sediment filter
(
http://www.waterfilters.net/culligan/wholehouse/replacement_filters/P5.htm).
When the pressure drops, you will know that you need to change the
sediment filter. When there is hardness in the water again, you will
know you need to change the water softening cartridge.
It always takes a practice run before you know exactly when to change
the filters, because all water and usage varies. The clear housings
will help you at least see how the filters are looking.
Thank you,
WaterFilters.NET