Heater size chart
Find your tank and the column that matches the coldest your room gets. The chart is for a water temperature of 78 °F (25.5 °C) and a tank with a lid, and shows the heater sizes to buy.
| Tank | Warm room73 °F / 23 °C | Normal room68 °F / 20 °C | Cold room63 °F / 17 °C |
|---|---|---|---|
| 5 gallon19 L | 25 W | 25 W | 50 W |
| 10 gallon38 L | 25 W | 50 W | 75 W |
| 15 gallon57 L | 50 W | 75 W | 150 W |
| 20 gallon76 L | 50 W | 100 W | 150 W |
| 29 gallon110 L | 75 W | 150 W | 250 W |
| 40 gallon151 L | 2 × 50 W | 2 × 100 W | 2 × 150 W |
| 55 gallon208 L | 2 × 75 W | 2 × 150 W | 2 × 250 W |
| 75 gallon284 L | 2 × 75 W | 2 × 150 W | 2 × 200 W |
| 90 gallon341 L | 2 × 75 W | 2 × 150 W | 2 × 250 W |
| 125 gallon473 L | 2 × 100 W | 2 × 200 W | 2 × 300 W |
For other temperatures, an open-topped tank, or a size not listed, use the heater size calculator.
The rule behind the chart
Heaters are sized by how many degrees they must lift the water above the room:
So the same 20-gallon tank needs 50 W in a warm flat, 100 W in a normal house, and 150 W or more in a cold basement. That is why a single "watts per gallon" figure, quoted without a room temperature, so often leads people wrong.
One heater or two
From about 40 gallons (150 litres) up, split the wattage across two heaters, one at each end of the tank. The heat spreads more evenly, and you have a backup whichever way a heater fails: if one stops, the other holds the temperature; if one sticks on, half the power heats the tank slowly enough for you to notice.
Types of heater
| Type | Good for | Keep in mind |
|---|---|---|
| Submersible glass or quartz | Most tanks; the standard choice | Can crack if it runs dry or is knocked by large fish |
| Shatterproof (titanium or plastic-cased) | Large or boisterous fish, turtles | Titanium models usually need a separate controller |
| Preset mini heater | Small betta and shrimp tanks in warm rooms | Fixed at about 78 °F; cannot be adjusted |
| In-line heater | Tanks with canister filters | Fits on the filter's return hose, out of sight |
| Substrate cable | Specialist planted tanks | Rarely needed today |
Whichever type you choose, an adjustable thermostat is worth having, and so is a separate thermometer at the opposite end of the tank from the heater.
Fitting and safety
- Place it in moving water, next to the filter intake or outflow, set at an angle so heat rising from the element does not fool its own thermostat.
- Keep it fully under water, and leave it for 20–30 minutes after installing before you plug it in, so the glass reaches water temperature.
- Unplug it before every water change, and wait a few minutes before the water level drops below it. Plug it back in only once it is covered again.
- Use a heater guard with large cichlids, plecos and other fish that could break it or burn themselves against it.
- Consider a temperature controller for tanks of 55 gallons and up, or valuable fish. It switches the power off if the heater's own thermostat sticks on, which is the most common way heaters kill fish.
- Replace heaters every few years. Thermostats wear out. A heater that leaves the temperature swinging by more than a degree or two is due for replacement.
Do you need a heater at all?
Tropical fish need steady warmth, usually 24–26 °C (75–79 °F). If your room stays in that range day and night all year, you can go without. Most homes do not, particularly at night, and a steady temperature matters as much as the right one: swings of several degrees in a day stress fish and bring on disease.
Fish kept without heaters in a normal home include fancy goldfish, White Cloud Mountain minnows and zebra danios. Axolotls need cool water and must never be heated.
Questions and answers
What size heater do I need for a 5 gallon tank?
A 25 W heater suits a 5-gallon tank in a normal room. Choose an adjustable one for a betta, which prefers 78–80 °F (25.5–27 °C).
What size heater for a 40 gallon tank?
About 200 W in a normal room: either one 200 W heater or, better, two 100 W heaters at opposite ends.
What size heater for a 75 gallon tank?
About 225 W for a 10 °F rise. Two 150 W heaters is the usual choice, which also covers a colder night.
Is it better to have a heater that is too big or too small?
Slightly too big is better than too small, because a small heater cannot hold the temperature on a cold night. But avoid going far over: a much oversized heater overheats the tank quickly if it sticks on.
How long does a heater take to warm a tank?
A correctly sized heater raises the temperature by roughly 1–2 °F (0.5–1 °C) an hour. A new tank filled with cool tap water usually reaches temperature within a day.