Aquarium Heaters: Do You Need One, What Size, and Which Type

Tropical fish and bettas need a heater; goldfish usually do not. University of Illinois vets give bettas 76 to 81°F, while the RSPCA gives goldfish 10 to 21°C and calls a heater optional. Aqueon’s rule is 5 watts per gallon up to 55 gallons, so a 20 gallon tank takes a 50 to 100 watt heater.

Which fish need a heater?

The two RSPCA pages differ on goldfish: the UK page allows cooler water than the Australian one. A thermometer in the tank tells you where yours sits. For details, see the betta setup and goldfish setup chapters.

What size heater for your tank?

Aqueon gives two ways to size a heater. Its rule of thumb is 5 watts per gallon for tanks of 55 gallons or smaller and 3 watts per gallon above 60 gallons. Its wattage guidelines give overlapping ranges:

Where ranges overlap, room temperature decides. Aqueon suggests a larger heater, or a second one, when the tank is in an especially cold room, on an exterior wall or near an outside door. Its heater tips are based on a room at 68 to 72°F.

Adjustable aquarium heaters by wattage

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Adjustable or preset?

A preset heater holds one factory temperature. An adjustable heater has a dial, so you can set it for your species, such as the betta range above. Either way, check the result with a separate thermometer and wait: Aqueon says to allow several hours before checking an adjustment, and to avoid sudden temperature changes that stress fish.

A heater guard, a plastic sleeve around the heater, keeps large fish and decor from touching it. Aqueon says to keep decorations away from the heating element.

Do aquarium heaters use a lot of electricity?

A worked example with a 100 watt heater: running nonstop for 24 hours, it would use 2.4 kilowatt-hours. At the EIA’s US average residential price for July 2026, 18.31 cents per kilowatt-hour, that is about 44 cents a day as an upper limit.

A heater’s thermostat switches it off once the water reaches the set temperature, so real use depends on how much warmer the tank is than the room. A tank in a warm room costs far less than one on a cold exterior wall. Your own utility rate may be higher or lower than the national average. To estimate your own cost, multiply the heater’s wattage by the hours it runs, divide by 1,000 for kilowatt-hours, and multiply by the rate on your electric bill.

Heater safety

Unplug the heater before a water change too, so it is never running while the water level drops below it, which Aqueon’s out-of-water warning is about. The cleaning chapter builds this into the routine. For light and timers, see aquarium lighting, and for everything else, the setup checklist.