It’s only 12 degrees outside and will likely drop further to below 10 degrees according to forecast.  To winter-proof my studio’s aquarium, I’ve added a layer of dark gravel in the tank to absorb more radiant heat during the day.  Also set up a few candles on the side to provide some active heating.  The thermometer in the tank is now at 20 degrees, hopefully the heat capacity of the water and sediment will keep the temperature high enough for my four Corydoras Sterbai to survive the cold night(s).