If you have an internal filter or canister filter, remove it after the power goes off. Once the power comes back on, clean it thoroughly and put it back in service. If you have an undergravel filter, try to keep it running - don't know if a battery powered airpump has enough grunt...
Bacteria in filters start to die off after 30-45 minutes. After 4 hours almost all bacteria are dead and they start to go toxic. If the power comes back on with the filter still connected all the toxins wash straight into the tank. By removing the filter and cleaning it this problem is eliminated.