Sometimes the salts in well water treatments are also bad for house plants. I was unpleasantly surprised when most of my houseplants took a nosedive after installing a purification system, then had the salesman offhandedly tell me, oh yeah, you should use other water for that. Luckily, using the treated water for outside watering has not been a problem - the untreated water turned everything (sidewalks, siding, stones) an unattractive rust color.
