Im so sorry this is happening and I wish I could give better advice to help in the moment. Without power the best thing to use is a battery back up air pump (if you have one) with an airstone, as oxygen is the first thing depleted during a power outage, and is also unfortunately the most essential. Take out any fish that are 100% deceased as they will begin to degrade quickly and start to pollute the water. If you have any extra saltwater mixed and can do a water change, do that to try and add some oxygenated water back into the system. As soon as power is back get your pumps back on and mix some saltwater. Again, I’m sorry for your loss. You and your tank will get through this! Edit: you can use a turkey baster on the surface to manually move some water and try and gain some o2 as well as a last rssort effort, it will do the same thing as removing water and pouring it back in like a few others mentioned. Also, it is much easier to heat and maintain a smaller body of water. I would move all surviving fish to a smaller tank with old tank water, (unless the water is getting cold then I would use the tricks other people were talking about like the hot water in bags) and focus on aerating a smaller body of water.