Aquarium heater failure: what to do when your fish tank is too cold

A cold tank usually means the heater has failed or can't keep up with a cold room. The most important thing to do is warm the water back up slowly, about 1°C (2°F) per hour, and then watch for ich over the next two to three weeks.

Fish are cold-blooded: their body temperature is the water's temperature, and everything from digestion to their immune system speeds up or slows down with it (Merck Veterinary Manual). Tropical fish are built for steady, warm water, usually 22–30°C (72–86°F) depending on the species. A slow fall of a few degrees is often survivable. The real danger is a big or fast change, and what comes after: chilled fish have weaker immune systems, and outbreaks of ich very often follow a cold snap.

How to identify it

The thermometer reads lower than usual. (This is why you need one, and should look at it daily.) The heater's indicator light is off when the water is cold, or the heater feels cool. Fish are slower, sit near the bottom and show less interest in food.

Common causes: the heater has failed (broken, unplugged, tripped or stuck off); it's too small for the tank or the room got much colder (a cold night, an open window, a basement fish room in winter); or there's a power failure.

Easily confused with: low oxygen or ammonia (fish also go quiet), or a heater stuck on, which overheats the tank instead (see fish tank too hot). A thermometer settles it.

When it's a problem

Early: a few degrees below normal; fish slow and less hungry. Fix the heater and warm slowly.
Serious: fish lying on the bottom, clamped fins, pale or dark colour, slow breathing. Warmth-loving fish (discus, rams, many plecos) losing balance.
Emergency: water reaching the mid-50s °F (about 13°C). NC State's vet school recommends warming the water at that point. Warm-water specialists like discus, rams and many plecos (including zebra plecos and other Hypancistrus) are much less tolerant; get them warm as soon as you safely can.
Normal, don't worry: NC State notes many tropical fish tolerate the low 60s or high 50s °F (about 14–17°C) for several hours. A small, slow dip that you catch and correct is usually fine.
Later: a week or two after the chill, white spots (ich), fungus or other infections can appear.

What to do right now

Check the temperature with a reliable thermometer. If you have two, check both.
Check the heater: is it plugged in, is the outlet working, is the breaker or power bar tripped, is the dial set right? Unplug it for 30 seconds and plug it back in.
If it's dead, replace it or move in a spare heater. If you don't have a spare, use the warming tips below and get one today.
Warm the tank slowly. Aim for about 1°C (2°F) per hour or slower. The Merck manual advises limiting temperature changes to about 1°C per hour during handling and transport.
Insulate the tank: put the lid on and wrap the sides and back with blankets, towels or foam. Leave a gap for air.
Don't feed until the tank is back to normal. Cold fish digest slowly, and uneaten food rots.
Mark your calendar to watch for white spots over the next 2–3 weeks.

What to do next

No spare heater? Safe ways to add warmth

Warm water in sealed bottles floated in the tank, swapped as they cool.
Small warm water changes: NC State suggests adding warm, dechlorinated water gradually, about 10% of the tank every hour or two, if you can't move the fish (NC State CVM). Check the new water with a thermometer; never pour in very hot water.
Warm the room with a space heater, used safely and well away from water.

Bring the temperature back, slowly

Once the heater works, set it to the tank's normal temperature and let it climb gradually. Most aquarium heaters raise water slowly on their own. Don't stack extra heaters to rush it.

After the cold: prevent the ich wave

Watch daily for white spots for 2–3 weeks. Ich develops slowly in cold water and speeds up as the tank warms (UF/IFAS).
Keep water clean: a 20–25% water change (temperature matched) once the tank is warm again.
Test ammonia and nitrite. Filter bacteria slow down in cold water, and a mini-cycle can follow.
Restart feeding lightly.
If ich appears, treat right away. See the ich guide.

Cautions

Shrimp and snails are usually more cold-tolerant than tropical fish, but they hate fast swings too.
Plecos and loaches from warm rivers, and scaleless fish, often show stress first.
Live plants may slow down but usually recover.
Heater safety: never put a heater in a tank while it's plugged in out of water, and let a heater sit in the water 15–30 minutes before plugging it in (check your heater's instructions). Glass heaters can crack.

Products that help

Digital thermometer with alarm: tells you the tank is cooling before the fish show it. A second thermometer lets you double-check.
Spare aquarium heater: swap it in the moment a heater dies.
Two smaller heaters per tank: if one fails, the other keeps the tank from crashing; if one sticks on, it's less likely to cook the tank.
Tank lid: cuts heat lost through evaporation.
Foam sheets or blankets for insulation: slow heat loss in a cold room or during a failure.
Ammonia and nitrite test kit: catches the mini-cycle that can follow a chill.
Temperature monitor with phone alert: warns you while you're away.

Keep on hand before trouble starts: a spare heater sized for your tank, a thermometer, and something to wrap the tank in.

How to prevent it

Size heaters for your coldest night, not an average day. A common rule of thumb (quoted from Aqueon by Build Your Aquarium) is about 5 watts per US gallon (about 1.3 W per litre) for tanks up to 55 gallons (210 litres), and about 3 W per gallon for larger tanks, more in cold rooms or near exterior walls.
Use two smaller heaters instead of one big one, with the combined wattage your tank needs, on opposite sides of the tank.
Keep a spare heater in the cupboard.
Check the thermometer every day, or use a digital thermometer or monitor with an alarm.
Use a lid and consider insulating the back and sides of tanks in cold rooms.
For a fish room, heating the room itself is often more reliable than many separate heaters.

Why it happened

Heater failure: they wear out, often after a few years.
Undersized heater for the tank or the room.
A cold room: basements, rooms by doors or windows, homes that drop the thermostat at night.
Open-top tanks lose heat faster through evaporation.
A power failure or tripped circuit. The Merck table of environmental hazards lists temperature swings from power or thermostat failure as a cause of sudden deaths and of infections from a weakened immune system.

When to get help

Get help if fish don't recover within a day of warming, if you see spots, fungus or sores in the following weeks, or if you lost several fish. A good local fish store can help choose and size a heater. A vet who sees fish can help if an infection follows. Honest limit: very warm-water species may not survive a long, deep chill, even with careful rewarming.

In the game

In Multiple Tank Syndrome, the Cold night card hits every tank, for both players: "the heaters can't keep up". Every tank drops 4°C, and fish that need it warmer die. The Backup heater card saves all your tanks. In real life, a spare heater and two heaters per tank are cheap insurance.