I use boiled filtered water. (.2 micron filter - don't ask me the brand - that is my husband's world & I stay out of it - only know he changes the filter by the calendar.)
In 2+ years it has been fine for me. But I only use the actual NeilMed saline packets - got some others once and they burned my sinuses - maybe not correctly buffered.
Going through the Simply Saline like crazy right now as I am fighting a cold.
Sue