What Size Aquarium Heater Do I Need? (Watts per Litre)

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.

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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.

Small tanks and shell-dweller tanks

The watts-per-litre rule gets awkward at the small end. A 20 litre shrimp or betta tub does not want a 30 watt heater doing the full arithmetic, because small volumes heat and overshoot fast. On my own small tubs I fit a little adjustable heater of 25 to 50 watts and lean on the thermometer rather than the sum, watching that it clicks off well before the water climbs past target.

Shell-dweller and Rift Lake tanks sit at the warm end of the range, and I run mine around 25 to 26°C. Warmer water holds less oxygen, so if you keep a heavily stocked cichlid tank warm, make sure the surface is moving well. A heater that is slightly oversized for the volume is not a problem here as long as it is adjustable and you have checked it against a real thermometer, because the risk with these tanks is a cold snap dragging a busy tank down, not a gentle heater running a touch warm.

What a stable heater actually saves you

The reason I bang on about sizing and thermometers is that temperature swings are a silent killer. A heater that struggles in a cold room, cycling hard and never quite holding, gives you a tank that is warm by day and cool by night. Fish read that as chronic stress. It suppresses the immune system and it is behind a good share of the fin rot, fungus and “mystery” losses that get blamed on water quality. Size the heater with a little headroom, split the load on bigger tanks, and read the water rather than the dial, and temperature simply stops being one of the things that can go wrong.

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.