Fish tank heater overheating: how to cool a cooked tank safely

If your tank is getting hotter and hotter, unplug the heater now and get more air into the water. Heaters usually fail by sticking "on", and a stuck heater can cook a tank full of fish in a few hours, especially a small tank.

The good news: if you catch it while the fish are still swimming, most of them can be saved. Cool the water steadily, not all at once, and keep the oxygen up while you do it.

How to identify it

The thermometer reads higher than the heater's setting. For example, the heater is set to 26 °C (79 °F) but the water is 31 °C (88 °F) and still rising.
The heater light stays on all the time, even though the water is already warm. (Some heaters have a light that stays on when they fail, some don't, so don't rely on the light alone.)
The glass feels warm to the back of your hand, warmer than usual.
Fish gasping at the surface, crowding near the filter outlet or air stone, breathing fast, or swimming in a jerky, restless way.
Late signs: fish lying on the bottom, floating, losing balance, pale or blotchy colour, or sudden deaths.

Check the reading with a second thermometer if you can. Cheap stick-on strip thermometers are often off by 1–2 °C.

When it's a problem

Normal: most tropical tanks sit between 24 and 28 °C (75–82 °F). A swing of half a degree over the day is fine.
Early warning: 1–2 °C (2–4 °F) above your setting and climbing. Fish may look fine. Act now and you will probably lose nothing.
Serious: about 30–32 °C (86–90 °F) for ordinary tropical fish. Warm water holds less oxygen, while the fish need more of it, so gasping starts here.
Emergency: mid-30s °C (mid-90s °F) or fish lying on the bottom or floating. Deaths can happen within hours.

Some fish, such as discus and many plecos, like warmer water than average, but no aquarium fish is safe with a heater that won't switch off.

What to do right now

Dry your hands and unplug the heater at the wall. Leave it in the water for now. A hot glass heater can crack if you pull it out into cool air; most makers say to wait 15–30 minutes before lifting it out.
Add oxygen. Turn on an air pump and air stone, lower the filter outlet so it splashes the surface, or point a powerhead at the top of the water. This is the single most important step, because hot water holds much less oxygen.
Take the lid off and turn the lights off. Lights add heat. An open top lets heat escape.
Point a fan across the water surface. Evaporation cools the tank surprisingly well.
Float sealed bottles of ice or frozen water in the tank. Never tip loose ice straight in, and never use ice made from untreated tap water.
Do small water changes with slightly cooler, dechlorinated water, about 10–20% at a time, with water only a few degrees cooler than the tank.
Cool it steadily. Aim for about 1–2 °C (2–4 °F) per hour. Experts disagree on the exact speed: overheated fish need cooling faster than chilled fish need warming, but a sudden big drop is a second shock. If the water is at a deadly temperature, cool faster at first, then slow down as you get near the normal range.
Stop feeding until the temperature is back to normal. Digestion uses oxygen.

What to do next

Replace the heater. Once a heater has stuck on, don't trust it again. Throw out any heater with a cracked glass tube, a damaged cord or water inside.
Keep the aeration running for 1–2 days after the temperature is back to normal.
Test ammonia and nitrite for the next week. Heat can kill some of the filter's helpful bacteria, and dead fish rot fast in warm water. See ammonia and nitrite.
Remove any dead fish quickly and check behind decorations.
Watch the survivors for 1–2 weeks. Heat-stressed fish are more likely to get infections such as columnaris or fin rot.
If your room can't stay cool enough without a heater, run the tank without one until the replacement arrives, and watch for the opposite problem: see a tank that's too cold.

Products that help

External temperature controller: a plug-in box with its own probe that cuts power to the heater if the water gets too warm. Use it on every heated tank, set a few degrees above the heater's setting.
Two smaller heaters instead of one big one: if one sticks on, it isn't strong enough to cook the tank on its own; if one dies, the other keeps the tank warm.
Digital thermometer with a probe (ideally one with a high-temperature alarm): more accurate than stick-on strips, and the alarm tells you before the fish do.
Heater guard: a plastic cage that stops plecos and loaches resting on the glass and getting burned.
Battery or mains air pump and air stone: for emergency oxygen.
Keep on hand: a spare heater, a few sealed water bottles in the freezer in summer, water conditioner, and an ammonia test kit.

How to prevent it

Use an external controller. This is the best protection there is. The heater's own thermostat becomes the first line of defence and the controller becomes the backup.
Size heaters modestly. Many packages suggest about 5 watts per US gallon (about 1.3 W per litre). Some experienced keepers think that is far too much, and suggest a total of no more than about 3 W per gallon split between two heaters. The right number depends on how cold your room gets. Too big a heater turns on and off more often, wears out sooner, and does more damage when it sticks.
Check a separate thermometer every day, ideally at feeding time, so it becomes a habit.
Never run a heater out of water. Unplug it before water changes, before draining the tank, and when the water level drops from evaporation. A heater left on in air gets very hot and can crack its glass tube. A cracked heater is also an electrical hazard: see aquarium electrical safety.
Keep the water level above the heater's "minimum" line and top up evaporation regularly.
Replace heaters every few years, before they get old and unreliable.

Why it happened

Most heaters have a simple switch inside (the thermostat) that turns the heating element on and off. With age, the contacts can weld themselves shut, so the heater stays on no matter how hot the water gets. A heater that is too big for the tank makes this worse because it switches more often and heats faster. A heater left running while the water level drops, or in an empty tank, can overheat and crack. Summer heat, strong lights or a sunny window add to the problem; see heatwave.

When to get help

If fish are lying on their sides or floating and the tank won't cool, call a local fish store or an experienced fishkeeper for help right away. Some stores will hold fish for you.
If fish start showing sores, white patches or ragged fins in the days afterwards, contact a fish veterinarian (search "aquatic vet" or "fish vet" near you).
If the heater glass is cracked and you feel a tingle in the water, stop and follow electrical safety first.

In the game

Multiple Tank Syndrome doesn't have a stuck heater card yet; the closest is the "Backup heater" card.