The short answer
As a rule of thumb, aquarium heaters are sized at roughly 1 watt per litre, so a 100 litre tank wants a heater of about 100 to 150 watts. The exact figure depends on how warm your room is and how big a gap you need to bridge between room temperature and your target tank temperature.
How to size it properly
The 1 W/L guide assumes a reasonably heated room. The real job of a heater is closing the gap between ambient temperature and the ~24–26 °C most tropical fish want:
- Warm, stable room: a 100 W heater is plenty for 100 L.
- Cooler room, or a big gap to close: step up to 150 W so the heater isn’t running flat out.
- Cold room or draughty spot: 150 W is the safer pick, and two smaller heaters spread the risk.
Erring slightly high is usually smart — a heater that only ever runs at part load lasts longer and recovers faster after a water change than one straining at its limit.
Getting the placement right
Wattage only works if water flows past the heater. Mount it near the filter outlet or an area of good circulation so warmed water is carried around the tank rather than pooling in one spot. Keep it fully submerged to the marked line, and give it a minute to adjust after a water change before you judge the temperature.
For picking a reliable model, see our best aquarium heater round-up and the how to choose an aquarium heater guide. You can also browse all aquarium heaters, or check the sibling questions what size heater do I need? and do I need a heater at all?