I purchased this "100gal 3 stage unit" when I started the hobby a year ago, and it's still running at 0 TDS! (Of course the DI part was replaced twice already). I estimated I've done 200gal+ water change but TDS still reads 0 as of today. Is there any hidden stuff that I don't know? Or it's still good? My tap water is Hard Rock (700TDS average).