Most car alternator produce between 40 to 120 amps. Let say your car produces 70 amp X 12 volts = 840 Watt. you wont have available those 840 watts because some of that power will be used to power up the engine ECU, sensors, lights, etc. Then you will loose more power when you convert from DC to AC.
In a desperate situation if you keep the A/C , lighs and radio off in your car you will have some power available to run a MAG DRIVE circulation pump and maybe some other stuff but not much before you overload the alternator, then the battery car will become the power source until losses the energy and your car and aquarium will shut off. It could work but you have to power the most essential stuff .
Better preparation leads to better results. I will have a 2000 watt generator. they are small and not very loud.
Here in Florida is a must. hurricanes can leave us without electricity for several days/weeks.