An ATS will pretty much keep phosphates and nitrates at zero, while supporting a healthy fauna population. Couple that with apropriate bio-filtration and you have a tank that can run for a long period of time with out a w/c. All this is presumably in a sump, which is going to help keep disolved oxygen levels where they need to be, and topping off with dechlorinated tap will replace carbonates, etc neccesary to keep ph, calc, mag (and so on) at good levels.
I believe all test measure in ppb or ppm
I believe all test measure in ppb or ppm

