Why overheating is an oxygen emergency too
A heater that sticks on does two kinds of damage at once. The rising temperature speeds up the fish's metabolism, so each fish needs more oxygen just as the warmer water physically holds less of it. That squeeze is why fish in an overheating tank gather at the surface gasping, even before the heat itself reaches a lethal level. It is also why your first moves, after cutting the heater's power, are about air rather than ice: maximum surface movement and an air stone put oxygen back into water that is temporarily poor at holding it. Switch the heater off at the wall rather than trusting its dial, because a thermostat that has failed once is already lying to you.
Cooling gradually, never suddenly
Fish are harmed by rapid temperature swings in either direction, so the goal is a slow glide back to normal over an hour or more, not a plunge. Lift or prop the lid open so heat can leave, and point a room fan across the water surface if the room is cooler than the tank. Float sealed bottles or bags of cool — not icy — water in the tank, and swap them out as they warm so the drop stays gentle and steady. Check the thermometer every few minutes; if the temperature is barely moving and fish are still gasping at the surface, the aeration matters more than faster cooling. Never pour in jugs of cold tap water: the sudden chill, plus untreated chlorine, can do more damage in a minute than the heat did in an hour.
Reading the fish over the following day
Once the normal range is restored, watch behavior rather than the thermometer. Fish that return to mid-water, spread out and feed at the next meal have usually come through well. Fish that stay clamped at the surface, hang listlessly or refuse food into the next day may be dealing with the after-effects of oxygen deprivation or heat stress, and weakened fish sometimes show illness in the days that follow an overheating event. Keep the water especially clean, test it, and avoid any other changes — no new fish, no rearranging — while the tank settles. A fish that is still gasping or has lost its balance after temperatures normalize needs an aquatic vet's advice promptly, not more home intervention.
Replacing the heater without a repeat
Do not put the failed heater back on probation; a stuck thermostat rarely heals. Before the replacement goes in the tank, run it in a bucket of water with a separate thermometer and confirm it switches off at the set temperature, because that bucket test is where a faulty new unit reveals itself harmlessly. Fit the new heater, keep the independent thermometer you relied on during the emergency permanently in the tank, and consider a controller that cuts power if the water passes a limit you set — a second guard against the same failure. While the water glides back toward its normal range, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.