Depends on how long the power goes out and the size of your tank and the amount of fish you have in it.
In most cases, you can get by with just a battery backup running the air stones provided the tank isn't overstocked and the power will return within a few hours.
On the other hand, if your farming something like 100 full size talapia in a 300 gallon stock tank... Well you better have a generator ready and fuel to go with it.
More details would help us provide better answers to the question.
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