When I lived in Wisconsin, I kept styrofoam panels cut to fit the sides and fronts of all my tanks (about 15 tanks of warm water species), and a roll of tape to attach the panels during power outages in winter.
I usually already had the back, some sides, and the bottoms covered with styrofoam panels, taped on, to conserve energy anyway.

Here in Panama I don't worry about heat, but I had a pump fail, and it took me almost a month to get another because of my remote location, (I now have back ups) so I would remove at least a couple gallons of water every daylight hour, and replace it gradually with new water, with the method below helping to agitate the surface, and overflowing some to the sump to keep the bacterial colony going.

I only lost one fish during the month long lapse, it was a large and more of a rheophillic species than the others, requiring highly oxygenated water.
Another reason to always stock lightly.