For most tropical tanks in a normally heated room, size an aquarium heater at roughly 1 to 1.5 watts per litre. A 60 litre tank wants somewhere around a 75 watt heater, and a 100 litre tank a 100 to 150 watt one. That guide keeps the water at a stable tropical temperature without a heater that runs flat out or one that never keeps up.
When to go higher than the rule
The watts-per-litre rule assumes the tank sits in a room that stays reasonably warm. Push the wattage up towards the higher end, or beyond it, if any of these apply:
- A cold room. A garage, conservatory or unheated spare room asks more of the heater, so allow closer to 2 watts per litre.
- A big gap to reach. The further above room temperature you need to hold the water, the more power it takes.
- A large tank. Above roughly 200 litres, a single heater is slow to recover after a water change, and this is where a second unit earns its place.
Two smaller heaters beat one big one
On any tank past about 100 litres I fit two smaller heaters rather than one large one, each sized so the pair adds up to the target wattage. Two reasons. If one fails on, the smaller unit cannot cook the whole tank before you notice. If one fails off, the other holds enough warmth to keep the fish alive until you replace it. Heaters fail, and they fail in both directions, so splitting the load is cheap insurance.
Read the water, not the dial
Whatever heater you fit, do not trust the dial printed on it. Fit a separate thermometer at the far end of the tank from the heater and read that. Set the heater, leave it a day, then adjust to the thermometer. Most tropical community fish want a stable temperature between 23 and 26°C, and what harms them is not being a degree off the care sheet but daily swings, so keep tanks away from radiators, direct sun and draughty windows.
If you want a wattage matched to your exact tank volume, the heater calculator does the arithmetic. For where temperature sits among the other readings that keep fish healthy, see the aquarium water parameters guide.