The heater is going to suck that juice out of the battery. If you can take that off you will likely run 3x longer.Not that this is helpful at the moment of crisis, but maybe for the future: I have put all the essentials (heater + return pump) on a UPS. Generator is better, of course, but hard in an apartment. I have always a battery-operated pump with an airstone sitting in case of longer power outages.
In an apartment, I would think it doesn't get to cold. Coral can handle cooling pretty well. Upper 60's/70, Fish mid to lower 60's.