We had no power for 10 hours that day and had no issues with the fish but don't have coral at the moment either. I have been through 20+ hour outages with my tank when I did have corals and lots of fish in the middle of winter. It's all about being prepared. A cordless drill with a beater attachment from the kitchen can whip in a lot of air into the water in a few minutes of run time, or a battery air stone. The biggest issue for me was the water temperature drop as its an old farm house with poor insulation. I luckily have a gas stove that will still run with no power and would fill a big soup pot arm the water and return it to the tank.
Or for my new setup the 4 dedicated tank circuits are all wired to a generator panel that should keep it running for quite a while, just turn off the Mhs.