R/O water storage & TDS

[Lance]

I keep R/O water stored in a 30 gal Rubbermaid container. This water is used for salt mixing, topping off sump, etc. I currently have 3 tanks set up: a 90 gal + 54 gal sump, 70gal, and a 20 gal. Between water changes, top off, etc. I go through this 30 gal container in 6-7 days, then fill 'er up again. Today I checked TDS in the container with the meter and it read 121ppm!! (I figured the R/O unit was not working, so I made up some R/O water and tested it: it read 1.0ppm (unit works fine). I then tested our tap water which read 12ppm. How is it possible for TDS to rise that high in 1 week? The storage container is rinsed out every week when I empty it so it is not a build-up of crud on the bottom. Any thoughts?