Supply the skimmers with a consistent volume of water,
Incorporate a prefilter,
Have a “working amount of water” (the amount it takes to “work” the tank, pumping water into the tank to a desired level, so that water will flow out, and
Have the needed buffer, in case the electricity goes out and the tank drains down to the lowest point of the overflow(s).
DESIGN AND FUNCTION OF THE SUMP BOX

It is desirable to have two separate areas in the sump. They are separated with a piece of Plexiglas that is high enough to keep the water separated while the sump is in operation, but not totally separate, so the sump can fill up completely in case of a power outage. We will refer to one side as “dirty” (water from the tank), and the other as “clean” (water from the skimmer and the prefilter).

You need an area where the water can be controlled and directed. This is done with a sump box. One side is water coming from the tank, going to the skimmer. The other side is water coming from the skimmer through the prefilter, going to the main pump and back into the tank. Gravity drains water from the tank, into one side of the sump. The skimmer pump (on this “dirty” side) pumps this water into the skimmer. The water then drains by gravity into the prefilter (“clean”) side of the sump box, collects there, and gets pumped back into the tank by the main pump.