My statement was not that TDS being low in tap water is bad, but that i t "isn't necessarily good", by which I meant that it isn't necessarily usable as is. Certainly it helps a DI last longer.
Some people assume low TDS tap water is better "as is" than high TDS tap water, and that isn't true, and that's the point I wanted to make.
For further discussion...
TDS is not a measure of the suitability of water. It might be 1000 ppm TDS and be fine, or 1 ppm TDS and be bad, depending on what the TDS is caused by.
If you somehow have 1 ppm copper in your incoming water from your own pipes (some people do, that is not above the EPA limit; it is not from the source water but your pipes) and an RO drops the TDS from 100 to 2 ppm TDS, it might still contain 20 ppb copper, and I would not want that much copper in my top off and water change water. If you started with 2 ppm TDS source water from an ultrapure lake, and ran it through your pipes, it might have way higher than 20 ppb copper and still be 2 ppm TDS.
Thanks for the clarification Randy. That makes perfect sense. Apologies, I phrased my question poorly at the start.
