Ironically I JUST posted my own thread yesterday on calibration issues with Fritz. When I first got the bottle 7 months ago, calibration was great. Stored in the dark at 62-68F, closed, no leaks, now apparently when "calibrated" to 35ppt/1.0264, it makes my RO read negative by a good margin. Currently assuming my tank has been ~.003 higher due to it, as I hadn't checked against RO in a while until last week. smh.