Karachi, 8–10 hours of outages in the bad months, four tanks, and I have not lost one in six years. Here is what actually matters, in order, because most advice on this gets the priority wrong.
1. Oxygen first, filtration second. People panic about the filter stopping and the fish are actually dying of low oxygen. A battery-operated air pump is Rs 1,500–2,500 and it is the single highest-value purchase. If you can only afford one thing, this is it.
2. The bacteria do not die as fast as you think — but they die. Your filter media holds the colony and they are aerobic. In a canister the water goes stagnant quickly and it turns anoxic in an hour or two; a sponge filter or an HOB is more forgiving. The critical rule after a long outage: do NOT just switch the filter back on if it has been off more than about four hours. The water inside has gone foul, and turning it on dumps that straight into the tank. Take the media out, rinse it in tank water — never tap water, that chlorine kills what survived — and then restart. I have seen more tanks lost to switching the filter back on than to the outage itself.
3. Stop feeding. From the moment a long outage starts. Less waste, less ammonia, less oxygen demand. Fish are completely fine for several days without food; they are not fine in ammonia.
4. Heat is the real killer in summer, not the filter. A tank in a 40°C room with no circulation stratifies and the fish crowd at the surface. Warm water also holds less oxygen, so the two problems compound. What works: a wet towel over the top with air moving past it, and float a frozen bottle of water — a sealed bottle, not ice into the tank. Bring the temperature down slowly, two or three degrees at most.
5. UPS. Mine runs the air pumps on all four tanks plus one filter. A modest UPS does this for many hours because air pumps draw almost nothing. Do not try to run a heater off it, that is where your capacity disappears.
6. Stock lightly and this whole problem shrinks. A tank at half the stocking level survives an outage that kills a full one. This is the least popular advice in the hobby and the most effective.
What I lost my first tank to, in 2019: twelve-hour outage, came home, switched the canister filter straight back on, went to bed pleased with myself. Everything was dead by morning. Not the outage — me.
