not a hole in the overflow box siphon
a hole in the return line.
when your power goes out, the water that the return pump is trying to pump up to the tank drains back down into the sump. Since the return line from the pump into the tank is under the water level line in the display tank, when this happens it creates a siphon, so that the return line then starts drawing water out of the display tank back down into the sump.
To alleviate this, in your return line, just under the water level, drill a hole in the return line so that when the water level gets below the hole, it will suck air instead of water.
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