The short answer
Use roughly 1 watt per litre of water as your baseline — so a 100-litre tank needs about a 100 W heater. In a cold room, or for a bigger temperature lift, step up to 1.5 watts per litre. This simple rule gets you the right heater for almost any typical tropical tank.
The watts-per-litre guide
Here’s a quick reference for a normally heated home (aim for ~24–26°C):
- Up to 25 L — 25 W
- 50 L — 50 W
- 100 L — 100 W
- 150 L — 150–200 W
- 200 L — 200–300 W
If your tank sits in a cold room, a garage, or a conservatory that drops at night, size up toward 1.5 W/L so the heater isn’t fighting the room.
What changes the number
A few factors nudge you up or down:
- Room temperature: the colder the room, the harder the heater works — size up.
- Tank vs room gap: heating water from 15°C to 26°C needs more power than from 21°C.
- Lid: an open-top tank loses heat to evaporation, so it needs a bit more wattage. A lid helps.
- Big tanks: over 200 litres, consider two heaters sharing the load for safety and even heat.
Getting it right
Always pair the heater with a separate thermometer so you can verify the actual temperature, and give a new heater a day to stabilise before trusting it. Adjust in small steps.
For product picks across sizes, see the aquarium heaters hub and our best aquarium heater guide. If you’re still deciding whether you need one at all, read do I need a heater.