Does the aquarium actually have an overflow in it already? One of those powerheads is for circulation within the tank and the other is a return pump from the sump. The return pump returns water from the sump to the tank, the tank fills then overflows into the overflow weir and syphons down through a pipe back into the sump. If the power is cut the pump stops, the tank stope filling and the overflow into the weir stops, thus preventing a flood. Make sure your return nozzle (the water that is flowing back into y he tank) is above or at the water line, or make sur it has a hole drilled into it otherwise is your power is cut water WILL syphon down the return pipe and you will have a flood. Once you do some googling and research it'll all become clear (as mud lol)
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I come from the land down under.....