If you know when it's coming insulate the tank with some foam from HD and make sure the top is sealed. By doing so you won't loose much heat at all and everything will be fine. Aluminum foil would be a poor choice, although it provides excellent insulation against radiant heat transfer it does virtually nothing for conduction which is what the tank will experience.
I've had a few long power cuts and the most dangerous part was a result from lack of oxygen not temperature. It would be best to have a back up power supply or at very least a battery powered air pump.
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