uhm.. better to get a hydrometer and use that to check the SG(specific gravity aka salinity)
brackish tanks should be around 1.005-1.015. use that as a scale to replenish water lost from water changes.
for top off you can use tap or RO.
in a true brackish set up p[ants usually won't thrive unless they are from an estuary or other brackish water area.
as mentioned other hardy plants may survive in this .