I've had the large (2 gallon) container for about a month.  I tested from the tap - first hot water 058 dropping to 037 and then cold water from the tap 028 and climbed to 037. That's an odd situation so I did it again, same thing, I don't know why the water would do that.  From both those sources into the ZERO it came out at 000.  So these filters work and that's satisfying, but as to why the TDS count from the tap at 2 different readings confuses me.