My 60 gallon made it through 86 hours with no power during an ice storm. We wrapped it with old quilts to try to slow down the temperature decline. We spent hours trying to find some C batteries in the dark but finally got a battery-operated aerator going. Then we just didn't look in at all, or try to feed the fish, until the power came back on. The temperature in the house got down to about 50 (F), and the tank got down to about 63. Lost about 5 cherry barbs; the corydoras, tetras, and kribensis all survived!
Having a battery-powered aerator for emergency backup may have saved the day(s), so I highly recommend that everybody have at least one on hand.