Generally speaking, our goal is to manage in a relatively small closed system, an environment that mimics the natural environment of the animals and plants we are keeping — think, Great Barrier Reef (a very large, open system). Note though that wide variations appear to all “work” successfully.
On a natural reef, there is a constant flushing of the ambient water.
General rule is 10% of system water volume per month. Some reef keepers do a higher percentage more frequently.