What actually threatens the fish, and how fast
A healthy tank tolerates a few still hours better than most keepers fear, so the first rule is not to panic. With the filter and heater off, the two clocks that start ticking are temperature drift toward room temperature and oxygen depletion as fish and filter bacteria keep breathing without circulation. A large tank drifts slowly and holds more oxygen; a small, heavily stocked tank in a cold room is the urgent case. Warm-water fish in a winter outage need heat conservation most, while in a summer outage the temperature may barely move and oxygen becomes the main worry. Knowing which clock runs faster in your home tonight tells you where to spend your effort.
Holding heat and oxygen without power
Close the lid and leave it closed — every peek lets out the warm air layer above the water. Wrap the tank's sides and top in blankets, towels or sleeping bags, leaving the wrapped insulation in place for the whole outage rather than checking constantly. For oxygen, a battery-powered air pump with an air stone is the single most useful outage tool and worth owning before you need it. Without one, manual aeration works: every half hour or so, fill a jug from the tank and pour it back from a height so it splashes and mixes at the surface. Float sealed bottles of warm water against the glass only if the tank is getting genuinely cold, and change them before they turn into cold bottles cooling the tank from inside.
What not to do while the power is off
Do not feed the fish at all during an outage; digestion competes for the scarce oxygen, and uneaten food decays into ammonia that nothing is filtering. Do not start a big water change with cold or unconditioned tap water to freshen things up — the temperature and chemistry swing is worse than the still water the fish are already coping with. Avoid netting fish into buckets unless the tank itself becomes dangerous; a bucket has less water, less oxygen and swings temperature faster than the aquarium it came from. And resist opening the filter to clean it: the bacteria in there are dormant, not dead, and rinsing the media in tap water during the outage would do the damage the outage itself has not.
When the power comes back
As equipment restarts, confirm the filter is actually flowing — some filters lose their prime and run dry — and check the heater is cycling normally rather than stuck. Test the water over the following day or two, because the filter bacteria will have had a hungry, low-oxygen spell and a small ammonia rise sometimes follows; a modest water change with matched, conditioned water handles it. Watch feeding behavior at the first meals: fish that eat and swim normally have come through, while a fish left gasping or listless after conditions normalize deserves a closer look and veterinary advice if it does not recover. Outages are where a local fish-keeping network earns its keep, and the KinsLe Circle of Love connects your free account to nearby pet lovers who can lend the pump, the generator socket or the warm room your fish need.