Why heat and low oxygen arrive together
A summer outage attacks the tank from two directions at once. Warm water physically holds less dissolved oxygen than cool water, while the fish in it — warmer, more active, digesting faster — are using more of it. At the same time the filter has stopped, so the bacteria that process waste are themselves starved of flow and oxygen, and any uneaten food or waste in the tank begins consuming oxygen as it decays. This is why a tank that was perfectly stable at breakfast can be in trouble by a hot evening, and why your plan has two halves that must run together: bring the temperature down slowly, and keep the water's surface exchanging air the whole time.
Cooling without shocking the fish
Fish tolerate a warm tank far better than a sudden cold plunge, so every cooling step should be gradual. Close curtains or move a shade over any glass the sun touches, switch off tank lights that add heat, and float sealed bottles of frozen water — the seal matters, because loose ice made from treated tap water melts unknown water straight into the tank and drags the temperature down in lurches. A small fan playing across the open top cools by evaporation surprisingly well. Check the thermometer as you go and stop adding bottles once the fall steadies; you are aiming to hold the temperature, not win a race to the bottom. Never perform a big water change with cold water as a shortcut — a large, fast swing does more harm than the heat you were escaping.
Keeping oxygen moving while the filter is still
With the filter silent, the surface is your lifeline, because oxygen enters water where water meets air. Lift the lid or slide the cover glass back, securing a sheet of mesh or netting over the gap so a startled jumper does not leave the tank. A battery-powered air pump earns its price on exactly this day, so run it in the main body of the tank; if you do not have one, swish the surface gently by hand every so often, or lift and pour a jug of tank water back in from a height to break the surface and stir the layers. Keep the room's air moving past the open top as well, since still, saturated air above the water slows exchange.
Feeding stops, and watching becomes the job
Stop feeding for the whole outage. Food offered now is pollution within hours: uneaten, it decays and consumes oxygen, and even eaten, it becomes waste in a tank with no working filter. Resist topping up the water level with untreated water for the same reason. Your remaining task is observation — fish gathering at the surface, rapid gill movements and lethargy in normally active species are the signs the oxygen is running short, and they mean more aeration and more cooling bottles, in that order. When the power returns, restart everything, then watch the fish before feeding a small meal. Alone in a hot, silent room, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.