Oh, I was agreeing with your math beaslbob.
My point was how high can your fish/plants/algae tolerate of substance X. For some things it is definately not 200ppm. On top of natural filtration, water changes are still necessary and the amount depends on a number of factors I was listing above as well as others I can't predict. The flip-side is how low can a specific ppm go if you change out too much water and are not replenishing with enough of substance X that a plant needs.
Dirtydutch sounds like he is asking about a set up and forget tank, and although you can get away with filtration, I think routine water changes, the ocassional water testin, and good circulation is still important, unless you are talking about a completely closed micro ecosytem.