hi marea,
even one battery operated one per tank is fine. just standby more batteries.
if it's long power failure you can remove the water till there's only an inch or two.[depending on how big your fish is] the fish flapping will oxygenate the water. when power is restored you can put back the water in. remember to clean out your filters. the filter bacteria will be dead in one or two hours and foul the water when you restart the filter.
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