A small betta tank holds little water, and little water cools quickly when a heater stops in winter. A betta kept at 76 to 82 degrees Fahrenheit (24 to 28 degrees Celsius) feels that drop sooner than fish in a large community tank. Panic helps nothing here. The priorities run in a clear order: slow the cooling, protect oxygen, leave feeding alone, and then recover slowly when power returns. Work through the priorities in that order, and the fish gets calm, steady care instead of a series of sudden fixes.
Slow the cooling in the first hour
Wrap towels, blankets, or spare clothing around the sides and back of the tank, leaving the top covered by the lid. Keep the lid on at all times, because warm air escapes through an open top faster than through glass. Move any draught away from the tank where you can, for example by closing a nearby door or drawing curtains near a cold window. Check a thermometer every hour and write the reading down, so the trend is visible rather than guessed. Avoid lifting the lid repeatedly to look, because each look lets warm air out. A fish that sits quietly near the bottom in cool water is conserving energy, and quiet behaviour alone during a cut is expected rather than alarming.
Support oxygen without chilling the water
A stopped filter leaves the water still, and still water holds less oxygen near the fish. Stir the surface gently by hand every hour or so, using a clean cup to lift and pour water back from a short height. A battery-powered air pump with an air stone does the same job without repeated lid lifting, provided the pump was bought and the batteries were checked before the cut. Place the air stone near the surface rather than deep in the tank, so the bubbles move the surface layer that exchanges air. Stop feeding for the duration of the cut. Uneaten food decays in still water, uses oxygen as it decays, and fouls a tank that cannot filter the waste away. A healthy adult betta tolerates a day or two without food far better than it tolerates dirty, low-oxygen water.
Adding warmth only when the fall continues
If the thermometer keeps falling despite insulation, float a sealed bottle filled with warm tap water in the tank, and remove the bottle once the water inside it matches the tank temperature. Repeat with a fresh bottle only when needed, and check the thermometer after each bottle rather than adding several bottles at once. Never pour hot water directly into the tank, and never use water that feels hot to the hand, because a local blast of heat beside cold water stresses a fish more than the cold itself. Keep any candles, heaters, or other heat sources well away from towels and blankets around the tank. The goal is a slow, even hold on temperature, not a rapid climb back to the normal range during the cut.
When power returns, recover slowly
When power returns, confirm that the heater and the filter have restarted, and watch the temperature climb gradually on its own. Test the water for ammonia and nitrite before feeding, because a filter that sat still for hours may need time to work at full strength again. Feed a smaller amount than usual at the first feed after the cut, and resume the normal routine only when readings look steady and the fish is moving and breathing normally. If the fish lies on its side, gasps at the surface, or shows laboured breathing after power returns, seek advice from an aquatic vet promptly. Note the minimum temperature reached, the length of the cut, and what you did, so the next winter cut starts from experience rather than from scratch. KinsLe's Care Teams stores that outage note as a shared household task list, so the next cut follows the recorded steps instead of relying on memory.