Originally Posted by Lumpy
If you have a large fish tank on a stand, you can place a candle, (or use 2 if larger than 55g), inside the stand and close the doors, (if there are any). Our power was out for quite some time and that helped keep the water warm.
They'll hold temperature for a lot longer than 8 hours, that's not the issue.
They're planted - when the lights go off, the plants stop photosynthesizing and start giving off CO2.
With the power off, there's no surface agitation and eventually the fish start to suffocate due to lack of oxygen.