Slot = in the normal ranges and not changing from day to day, or at least, as little as possible.
So for me, Alk 10, CA 450, MG 1420, that’s where in the range my system likes. These do not change (much) regardless of time tested.
Stable environments allow the “unseen” stuff (bacterias, algaes and micro-fauna) to thrive in terms of population which keeps your hard surfaces clean and is available for corals to use.
In most cases, photosynthesis is the primary process, however corals (some more than others) have learned how to capture food from the water column.
It’s what you can’t see, that corals intake. So stability and maturity is king, or at least, in my way of thinking.