Continuous water changes?

[Anemone]

It does but raises a fundamental question about the theory of the system.

If we dialyse tank water against pure water, at best we divide waste to equal concentration between the pure water and tank water. Minerals and salt are likewise divided. So can this possibly use any less water or salt than changing the same amount of tank water? And can't the automatic salt mixer be used to mix up new tank water just as easily? If not even easier since we shouldn't have to worry about pH or ORP of the new water?

E.g., instead of dialysing a gallon of tank water against a gallon of RO water, changing out a half gallon of tank water for freshly made tank water should remove the same amount of waste, require the same amount of salt, and use half the water. What am I missing here?