What to look for in a 150 W heater
- Adjustable thermostat. A dial you can set to the degree. Preset heaters cannot be corrected if they run warm or cold.
- Fully submersible. Lay it at an angle near the filter outflow so warm water is carried around the tank.
- Automatic shut-off. It should switch itself off when it runs dry, for example during a water change.
- A separate thermometer. Never trust the dial alone. Check the real temperature at the far end of the tank.
How heater wattage is calculated
A heater has to replace the heat the tank loses to the room. That loss depends on two things: how much water there is, and how many degrees warmer than the room you want it. The long-standing rule, used by heater makers, is:
Large tanks lose less heat for each liter they hold, because they have less glass and surface area per liter. So the calculator uses 5 W per gallon up to 55 gallons and eases down to 3 W per gallon from 75 gallons up, the same adjustment Aqueon publishes for its heaters.
Worked example. A 29-gallon tank in a room that falls to 68 °F at night, kept at 78 °F, needs a 10 °F rise: 29 × 5 × (10 ÷ 10) = 145 W. The next size sold is 150 W.
If the same tank sat in a 62 °F basement, the rise would be 16 °F: 29 × 5 × 1.6 = 232 W, so a 250 W heater, or better, two of 150 W.
One heater or two?
For tanks of 40 gallons (150 L) and up, two smaller heaters are better than one big one:
- Even heat. One at each end avoids warm and cold zones in a long tank.
- Safety if one fails off. The other keeps the tank from crashing.
- Safety if one sticks on. Half the wattage heats the tank far more slowly, giving you time to notice.
Use the coldest room temperature
Size the heater for the coldest the room gets, not the average: a winter night with the heating off, or a holiday week with the thermostat turned down. A heater that is slightly too big simply switches on less often. A heater that is too small runs all the time and still lets the temperature sink.
A lid matters too. Evaporation carries off a lot of heat, so an open-topped tank needs about 15% more wattage, which the calculator adds when you untick the lid box.
Temperatures for common fish
| Fish | °F | °C |
|---|---|---|
| Most tropical community fish | 75–79 | 24–26 |
| Betta | 76–82 | 24–28 |
| Discus, German blue ram | 82–86 | 28–30 |
| Fancy goldfish | 65–74 | 18–23, usually no heater |
| White Cloud minnow, axolotl | 60–70 | 16–21, no heater |
Common mistakes
- Buying a preset heater for a cold room. Presets aim at about 78 °F (25.5 °C) and cannot be turned up.
- One oversized heater in a small tank. If it sticks on, it can cook the tank in hours. Stay near the calculated wattage.
- Placing the heater in still water. Put it beside the filter outflow so the warm water circulates.
- Trusting the dial. Heater dials are often 1–2 degrees off: set the temperature by a separate thermometer.
- Forgetting to unplug it during water changes. A heater running in air can crack or burn out.
Questions and answers
How many watts per gallon does an aquarium heater need?
Between 3 and 5 watts per US gallon for a 10 °F (5.6 °C) rise above room temperature: 5 W per gallon for tanks up to about 55 gallons and 3 W per gallon for bigger tanks. Colder rooms need proportionally more.
Can a heater be too big for a tank?
Yes. An oversized heater cycles on and off rapidly and, if its thermostat fails, overheats a small tank quickly. Choose the size the calculation gives, or split it across two heaters.
Do I need a heater at all?
If the room stays within the range your fish need all year, no. Goldfish, White Cloud minnows and axolotls are kept unheated. Most tropical fish and bettas need a heater in any home that cools below about 75 °F (24 °C).
Numbers for standard tanks
Have a standard tank? Its size, weight, heater and stocking are already worked out.
- 10 gallon tank
- 20 gallon long tank
- 29 gallon tank
- 40 gallon breeder tank
- 55 gallon tank
- 75 gallon tank
- 125 gallon tank
- All tank sizes