In the main tank you have some sort of overflow.
As the water level rises in the main tank from the the water being pumped from the sump below, it spills over into the overflow in the main tank.
that water then flows down through a tube back into the sump.
as soon as the pump in the sump/fuge stops working (gets unplugged, jammed, fails or whatever) then the water stops going into the main tank, the water level in the main tank stops rising, and water stops overflowing.
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