I went 13 days w/o power after an ice storm. My 65 gal
was over stocked with various varieties of Rainbow fish. No losses. If you have a gas water heater, change out five gallons of water 3x a day and replace with very warm water. Cover the tank with a blanket.
Another way is to fill Rubbermaid containers with very hot water and float them in the tank.
During the course of the day, pour pitchers of tank water through your filter to help the BB survive.