One of the issues facing any aquarium owner is how to keep fish waste to a minimum. This is particuarly true during a power outage because the filters you've depended on to keep the water clear and clean are not operating.
Step 1: Stop Feeding Your Fish
When fish are being shipped, especially across great distances, breeders will often stop feeding them for 2-3 days prior to their departure. This keeps fish waste to a minimum when they are in small bags.
Your fish can actually go without food for as long as a week if they are in good health. So during a power outage, stop feeding them. If the power comes back on in a day or two, they'll be no worse for the wear - and your water will be cleaner.
If you have an extended power failure and you must feed your fish eventually, feed them half as much as you normally would. If you're accustomed to feeding them twice a day, feed them only once a day.
Step 2: Depend on Chemical Filtration
When your filter isn't working, you can substitute a chemical substance to keep your water clean. One option is to use Prime, which eliminates chlorine, nitrates and nitrites. With angelfish, you must use this sparingly - if they begin to hover near the surface and gasp for air, they are being deprived of oxygen - and that can mean too many chemicals in their water. Replace the water with fresh water, and you've eliminated some of the chemicals.
Step 3: Water Changes
If you have ready access to water, the best way to keep the tanks clean and the water aerated is frequent water changes. With angelfish, try not to replace more than 33% of their water at any one time because they are much more sensitive to water fluctuations (temperature, Ph, and water quality) than many other types of fish.