I had a 3-day blackout a couple years ago. It happened to be below freezing outside at about this time of year. The best I could do at the time was hook up my inverter to the car and run a long extension cord in the house. Hooked up filters, heaters, and air pumps. Worked fine all except the heaters. They just werent enough to keep the tanks warm, even siphoning out water, heating on a gas stove, and putting back in. Temps dropped to the upper 50's in the tank and lost about 1/3rd of the fish. I am preparing to buy a generator, but havent quite yet. Actually might have to have an electrician out since it doesnt appear I have any means of disconnecting from the mains if I use a gen.
I assume by hydo your talking electric power. Around here the grid is all interconnected so even if you live by a nuke your still taking power from all sorts of different units in all directions. I actually work for the power co.