most of the small Britta filters (for home use ) are using weak acidic resins with some actif carbon ,this means that only the tempory hardness will be removed (the Ca and Mg bound to the HCO3) also some divalent metals as Ni,Cu,Fe ....will be captured ,this water is certainly not totaly deionised
If a total deionised water is needed a mixed bed filter is the only way or to by the so called distelled water what always means demineralised ,a membrane (RO) is also a solution
i know it quite well because i am selling those resins to all the big players and use a MB for the rinsing water (last bath ) when i develop my films (wetting agent)
most of the commercial chemicals contain complexing agents and normal tapwater (without particles ) is ok