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Aquarium temperature: how to heat and cool a fish tank

Not urgent

Fish are cold-blooded. Their body is the same temperature as the water, so the water sets how fast they digest food, grow and fight off disease. Get the temperature right and keep it steady, and you prevent a lot of problems before they start.

Most tropical community fish do well at about 24–26°C (75–79°F), which means a heater in almost every home. Goldfish and some other fish don't need one. Every species card on this site shows the fish's temperature range in °C: choose fish whose ranges overlap, set the heater inside that overlap, and check it with a thermometer. If your tank is in trouble right now, go to tank too hot, tank too cold or heater failure or heater stuck on.

How to check it

  • Use a thermometer you can trust. Don't rely on the dial on the heater. Heater dials can be out by a degree or more, and a heater can fail without any warning light.
  • Put the thermometer away from the heater, ideally at the other end of the tank, so it reads the water the fish live in, not the warm spot next to the heater.
  • Check it every day, at the same time, when you feed. It takes two seconds and is the easiest way to catch a dead or stuck heater.
  • Check it at different times of day for the first week, and again when the seasons change. A tank by a window, a radiator, an outside wall or a door can swing much more than you'd expect.
  • Look up each fish's range. Open the species card for every fish you keep and note its temperature range in °C. Write down the overlap: the range that suits all of them.

When it's a problem

  • Normal: the tank sits steadily inside every fish's range. A small, slow drift of a degree or so between day and night is normal, and happens in nature too.
  • Needs fixing this week: the temperature is outside one of your fish's ranges, the reading moves around a lot from day to day, or you have no thermometer at all.
  • Serious: a fast change of several degrees, for example after a big water change with cold or hot water. The Merck Veterinary Manual suggests about 1°C an hour as a safe maximum when moving fish between waters, and warns that sudden changes of a few degrees can weaken a fish's immune system. Ich often follows a chill.
  • Emergency: fish gasping at the surface in a hot tank, or lying still in a cold one. See tank too hot, heater stuck on or tank too cold.

What to do right now

  1. Read the thermometer. If you don't have one, buy one before anything else.
  2. Compare it with your fish's ranges on their species cards.
  3. If the tank is a little off, change the heater setting by a small step and check again in a few hours. Move it gradually over a day or two, not all at once.
  4. Feel the heater's light or check its indicator. If the tank is cold and the heater never comes on, or hot and it never goes off, it may be broken; unplug it and follow the emergency guides above.
  5. Match water-change water to the tank. Use a thermometer on the new water too, and get it within a degree or two of the tank before it goes in.

What to do next

Tropical or coldwater?

  • Tropical fish (tetras, most plecos, bettas, gouramis, most cichlids, corydoras) need a heater in most homes. RSPCA Australia recommends 24–26°C for typical tropical tanks, and points out that room temperature is usually too cold and too changeable on its own. Some species want it warmer than that and some cooler. The Merck manual notes that discus, for example, only thrive in a narrow band of temperature.
  • Goldfish and other "coldwater" fish don't need a heater indoors. Aquarium Science points out that goldfish cope with a very wide range of temperatures; the real risk for them is a fast change, not the number.
  • Your species cards are the guide. Each card's °C range tells you which group a fish falls into.

Choosing a heater

  • Size it by litres and by how cold your room gets. One heater maker's guide suggests about 1 watt per litre in a warm room, and about 1.5 watts per litre in a cooler room. So a 100-litre (26 US gallon) tank in a heated living room needs roughly a 100 W heater; the same tank in a cold basement needs more. The cold tank guide gives a similar rule of thumb.
  • Bigger isn't always better. Aquarium Science argues that the usual "3 to 5 watts per gallon" advice is more than most homes need, and that a big heater stuck on can overheat a tank fast. A heater that's a little small fails safe: the worst it can do is let the tank cool slowly.
  • Two smaller heaters are safer than one big one. Split the wattage between two heaters, one at each end of the tank. If one sticks on, it's too small to cook the tank on its own. If one dies, the other keeps the water from crashing. On long tanks, two heaters also spread the heat more evenly.
  • Consider a separate temperature controller for large or valuable tanks. It cuts the power to the heater at the temperature you set, so a heater stuck on can't run away. Aquarium Science calls these an excellent investment.
  • Choose a heater with a guard if you keep plecos, large cichlids or other fish that rest against it, so they can't burn themselves or crack the glass.

Where to put the heater

  • Where water flows past it, usually near the filter outlet or intake. Moving water carries heat round the tank and stops hot and cold spots.
  • Fully under water, unless the instructions say it may be partly out. Some heaters can crack if they run in air, or right after being lifted out hot.
  • Not buried in gravel or hidden behind rocks where flow is poor.
  • Unplug it before every water change, and wait the time the maker suggests before plugging it back in. Use a drip loop on the cable; see electrical safety.

Cooling a tank

Tanks overheat in a heat wave, in a sunny room, or because pumps and lights add heat. Warm water also holds less oxygen. Ways to cool, from easiest to most expensive:

  • Turn the lights off and close the curtains. Lights and sun both add heat.
  • Take the lid off, or swap it for mesh so heat can escape. Mesh keeps jumpers in; see jumping fish.
  • Blow a fan across the water surface. Evaporation carries heat away. The tank will lose water faster, so top it up with dechlorinated or RO water.
  • Add an air stone for extra oxygen while the tank is warm.
  • A chiller is the most effective option for a tank that is always too warm, but it's expensive.
  • Don't drop ice in the tank. It causes fast, uneven swings. If you need more help, follow the heat wave guide.

Matching fish by temperature

  • Find the overlap. If one fish's card says 22–26°C and another's says 25–30°C, the only temperature that suits both is 25–26°C. That's tight but workable. If the ranges don't overlap at all, the fish don't belong in the same tank.
  • Aim for the middle of the overlap, not the edge. That gives you a margin if the heater drifts or the room gets hot.
  • Don't keep goldfish with tropical fish. They need different temperatures, and goldfish make a lot of waste.

Products that help

  • Thermometer: a digital one with a probe, or a simple glass one. Stick-on strip thermometers are cheap but less accurate.
  • Submersible aquarium heater with an adjustable thermostat: two smaller ones for most tanks.
  • Heater guard: a plastic cage that stops fish touching the heater.
  • Temperature controller: plugs between the heater and the wall, and switches it off at the temperature you set.
  • Clip-on aquarium fan: cools by evaporation in summer.
  • Chiller: for tanks in hot rooms or coldwater species that need cool water all year.
  • Spare heater: kept in a drawer for when one fails.
  • Battery air pump: keeps oxygen up in a power cut, when the heater stops too.
  • Temperature monitor with alerts: warns your phone if the tank gets too hot or cold while you're out.

How to prevent problems

  • Read the thermometer every day when you feed.
  • Use two smaller heaters, or one heater plus a controller.
  • Keep tanks away from windows, radiators, heating vents and outside doors.
  • Match water-change water to the tank with a thermometer, not by hand.
  • Choose fish whose temperature ranges overlap, and set the heater in the middle of the overlap.
  • Check the heater before winter and the cooling plan before summer.
  • Keep a spare heater, and replace heaters that are old, cracked or unreliable.
  • Plan for going away; see leaving your fish.

Why it matters

Because fish can't warm or cool themselves, the wrong temperature affects everything. Too cold, and they slow down, stop eating and get sick more easily. Too warm, and they need more oxygen just when the water holds less, and ammonia becomes more toxic. Fast changes are the worst of all: the Merck Veterinary Manual lists temperature swings, power cuts and failed thermostats as common hazards, and notes that parasites like ich are more common when temperatures swing. Experts don't fully agree on how much small daily swings matter; Aquarium Science argues healthy fish handle more than people think. A steady temperature inside every fish's range is the safe target.

When to get help

Ask for help if your tank won't hold a steady temperature even with a working heater, if fish keep getting ich or seem stressed after temperature changes, or if you keep a species with a narrow range (discus are the classic example) and aren't sure what it needs. A local fish store, an aquarium club or a fish vet can help.

In the game

Temperature is one of the main rules of Multiple Tank Syndrome. Every fish card shows its range in °C, and every tank has a heater you can set between 18°C and 31°C. The heater can only move 2°C a turn, so you can't fix a tank in a hurry. A fish kept outside its range dies when the other player spots it and calls it out (in games without call-outs, you get one turn's warning to fix the water). The Cold night card drops every tank by 4°C and kills fish that need it warmer, unless you hold a Backup heater. The Cool-water tank goal asks for 10 or more fish happy at 20°C or cooler, and plants melt if the tank is outside their range. In a real tank, change the temperature more slowly than the game allows.

Sources

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