Generator would be most ideal, after that battery operated airpumps and lastly, water changes. Ive gotten through a short spell with frequent water changes and it seemed to help
If you know its coming like in a hurricane for instance. Do a fairly big water change and stop feeding the fish. Clean filters and change medias. Store water to do small water changes during the time the powers out. Keep the tank covered so that its dark. The fish will think its night and tend to stay at rest. If its a small tank then the battery operated air pumps arent a bad idea. But for big tanks or multiple tanks heres what I do. I have a power inverter hooked up to my battery. I just run the car for awhile to keep the battery charged and have a powerhead near the surface of the water to keep gas exchange going somewhat. Some people go the route of deep cycle marine batteries but Im in an apartment and thats not really an option for me. Clear out the water in any filters to avoid hydrogen sulfide build ups due to bacteria die offs and be ready to do another water change when the power comes back on. There is a sticky on this subject too.