A dead heater discovered in the morning feels like a power cut, and treating it as one leads owners wrong. The electricity never failed, the filter never stopped and the water has simply been cooling at the room's pace since the heater gave up during the night. Your thermometer reading, taken before anything else, tells you how serious the morning actually is: a small drop asks for calm replacement, while a large one asks for insulation first and patience throughout.
Slow the Cooling While You Work
Your first moves defend whatever warmth remains. Blankets or towels wrap around the glass on every exposed side, the lid stays shut, and the room door closes against draughts. Feeding pauses, since fish in cooling water digest slowly and uneaten food helps nobody. If the temperature has fallen seriously, sealed containers of warm water float in the tank and share their heat gently, each container swapped for a freshly warmed one as it cools, with the thermometer checked between swaps. The containers hold warm water only, never hot, because a floating bottle should nudge the tank's temperature rather than create a scalding pocket beside the glass.
Fit the Replacement Heater Properly
Your replacement gets chosen for the tank it must heat, following the sizing guidance on its packaging for the water volume involved rather than whatever wattage happens to be on the shelf. The failed unit comes out with its supply switched off, and the new heater takes a position where filter flow moves water across it, spreading warmth instead of pooling heat in one corner. Depth marks and immersion instructions from the maker govern how far the unit sits in the water, its cable drops in a downward loop, and a guard fits over the heater before the supply switches on, shielding fish that rest against warm surfaces and plants that lean. A heater whose cable cannot reach its socket without tension needs a different position rather than a stretched cable pulled tight across the tank's corner.
Warm the Water Back in Stages
Your patience during the climb matters as much as the insulation did during the fall. The heater's own dial is a setting, not a measurement, so a separate thermometer in the tank reports what the fish actually feel. You set the target the tank normally runs at, then confirm over the following hours that the reading rises slowly and steadies there, adjusting in small steps and waiting between adjustments rather than chasing the number. Kettles, hot tap water and jugs from the tap never enter the tank at any stage, since a sudden warm wave shocks fish that survived the cold perfectly well. Feeding resumes lightly once the normal temperature has held steady, and the fish get watched through that first day for the sluggishness or surface-hanging that says the cold took more out of them than the thermometer showed.
Stop the Next Failure Hiding Until Morning
Your defence against a repeat is a reading taken daily. A thermometer checked during the morning feed catches a heater that died in the night while the drop is still small, and a spare heater kept dry in the cupboard turns the next failure into an errand rather than an emergency. Note the failed unit's age and model beside the tank's records, since a pattern of early failures is information worth having. KinsLe keeps the tank's target temperature and the staged warming steps visible to the whole household through Family Sharing, so whoever finds a cold tank next time starts the same careful sequence.