A filter is not a water cleaner so much as a house for bacteria, and those bacteria need oxygen. Six hours without power and a good part of the colony dies, which is why tanks crash after a long outage rather than during it.
What people do:
- UPS or inverter running the filter and air pump, which is the proper answer
- Battery air pump as a backup
- Manually stirring the surface, which is better than nothing
- Cutting feeding during long outages so there is less waste to process
- Fewer fish than the tank could theoretically hold
The other half is heat: no fan, no cooler, and a tank in a Karachi flat climbs fast.
What is your setup, how long are your outages, and have you lost a tank to one? What did you change afterwards?