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Randall James has it almost exactly correct and quite clear/concise in answer to your post, Rebecca. Much thanks James!
The only clarification here is that new and slightly cooler seawater is sent to the main display, not the sump. And with mere partial water changes (10, 20% etc) it will sink in the greater volume of the main display and displace aged older water to the sump which will, as Randall states, "overfill". Indeed... overflow the drain as intended. And that drain is to be at or only slightly higher than the float switch that maintains the sump level with evaporation top off as described above.
As far as the chap that said putting the overflow hole halfway between the sump floor and running water level/surface... there is some kinf of misunderstanding or mistake. It's incorrect or not practical if even workable.
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"If you give a man a fish, he eats for a day... but if you teach a man to fish, he eats for a lifetime."
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