A long power cut does not turn an aquarium off in one way. Heat stops, filtration stops, and surface movement stops, while the fish keep breathing and the waste in the water keeps breaking down. The goal during the cut is stability rather than normal running: hold temperature, keep oxygen entering the water, protect the filter colony, and avoid adding waste. Work the priorities in order, check the tank on a steady rhythm, and save careful attention for the restart, which is where hurried handling causes losses that the cut itself did not cause.
Insulate first and leave the lid closed
Wrap towels, blankets, or spare clothing around the sides, back, and front of the tank, and keep the lid closed throughout the cut. A larger tank cools more slowly than a small one, yet every tank loses heat through glass and through an open top. Read a thermometer every hour or two and write each reading with the time, so a falling trend shows early. Keep the room as steady as you can by closing doors and curtains near the tank. Avoid floating bottles of warm water as a routine move in a community tank, because mixed species tolerate different rates of change, and an uneven warm patch helps little while unsettling many fish.
Keep oxygen moving while the filter sleeps
Oxygen enters water at the surface, and a still surface in a closed tank exchanges air slowly. Run a battery-powered air pump with an air stone when you have one, placing the stone where bubbles break the surface layer. Without a pump, lift a cup of tank water and pour it back from a short height every hour, working gently so sleeping fish are not startled by each visit. Watch the fish rather than the clock alone: fish gathered at the surface, rapid gill movement across the tank, or gasping at the top call for more frequent surface movement and a prompt call to an aquatic vet or an experienced local shop for advice if the signs continue. Stop feeding for the whole cut. Food eaten in a warm, filtered tank becomes waste that a working filter handles. Food eaten during a cut becomes waste that sits in still water and consumes oxygen as it decays.
Protect the filter colony while power is off
The bacteria that clean aquarium water live largely in the filter media, and those bacteria need oxygen and moisture. In a long cut, remove the media from an external filter that has sat still for many hours and keep the media submerged in a bucket of tank water, so the colony stays wet rather than drying or sitting in stagnant filter water. Do not rinse media in tap water, and do not replace media during an outage. Keep the tank itself covered and dark where practical, because darkness calms fish and lowers activity, and lower activity lowers oxygen demand.
Restart carefully and test before feeding
When power returns, confirm that the heater, the filter, and any air pump have restarted, and listen for a filter that primes poorly or rattles dry. Return any media kept in tank water to the filter promptly. Test the water for ammonia and nitrite before feeding, and change a modest portion of water with treated water at a matching temperature when readings show waste has built up. Feed lightly at the first feed, watch every fish feed and breathe, and return to the normal routine only over the following day or two as readings settle. Fish that gasp, cling to the surface, or show laboured breathing after the restart need advice from an aquatic vet. Write down the length of the cut, the minimum temperature noted, and the test results after the restart. Family Sharing through KinsLe means a chosen relative can be given access to the tank's profile.