That's scary. I really hate the thought of having my house burnt down. All your possessions and memories just gone in an instant. Same goes for natural disasters.
In a fire situation I'd get my dog out first then I'd go back in for the fish. I'd bag them and place them in a cooler/whatever container I could find that would fit the bagged fish. My human family would manage to escape themselves.
It would be easy enough to get supplies for them to survive.
However in a natural disaster such as a hurricane, tornado, earthquake, flood, etc it would be harder to care for the fish. Especially if the power lines were cut and the water became contaminated. You wouldn't be able to pop out to Petco and buy some water conditioner/new tank items either.
It's really a terrifying thought. And it's something I'd never really given thought to before this thread.