Rinse your frozen in a fine mesh net or sock in RO water before it goes in the tank. I tested some water from a cube once and I can't remember the number but phosphates were like 2.5 or something crazy like that.
Also, not to doubt the girl's reading, but I have never seen an RO system that put out 0 TDS. I think that's why you can drink RO water, but shouldn't drink RO/DI water because -everything- is filtered out of it. There's always something left from RO-only. My current setup has TDS of ~120 from the tap, .04 from the RO and .00 from the DI. Everything builds up.
One thing I think Daryl mentioned...it takes a while for the system to run at 00 when it starts up. Not sure why but my DI stage outputs sometimes .09 TDS for about 30 seconds when it starts up, I suppose the water in the chamber releases some of the trapped solids, I'm not sure, but I run the system until the meter shows 00 and THEN start filling my topoff tub. You might be doing the same thing with the small topoffs. Just thinking out loud here. I have a potable meter and check my reservoir every so often, and the inline meter keeps tabs on the filters and membrane.
One question for everyone, I haven't added a pressure gauge yet to my system, but it seems to be a good idea...what exactly would it tell me?