Forrest,
Unless you leave the windows open, your apartment should retain most of the heat (assuming non-drafty windows). If it is a newer building, most probably you have energy efficient windows (touch them...they're efficient if they aren't that cold). Even better.
Your 5 gallon with the angel "should" be fine with the heater. 5 gallons is pretty small and won't take much of a heater to maintain the temperature (the differencial should be only be an additional 5-10 degrees compared to normal).
If, for some reason, the thermometer is showing that the 5g tank is also getting much cooler, carefully (electricity), wrap the tank up in a blanket.