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How to Choose Aquarium Heater Wattage

Room maths for UK homes—not guesswork from the box end-cap

Aquarist Network Editorial9 min read10 May 2026

Aquarium heater wattage label beside tank volume calculation notes

Wattage is room maths, not guesswork

Aquarium heaters replace heat lost to the room. A tank in a warm lounge needs less power than the same tank in a conservatory overnight in February.

A keeper running a 60-litre tank in an unheated conservatory said their 50 W heater never switched off in February—the upgrade to 100 W finally held 25°C when outside dropped below freezing.

Rules of thumb help you shop; thermometer logs confirm you chose correctly.

The one-watt-per-litre starting point

Begin with roughly one watt per litre of water, then add twenty-five percent if the tank sits below eighteen degrees Celsius room average or against an external wall. For 90 litres, target 90–100 W; consider 150 W if the room is chilly.

What matters most in heater sizing

Headroom beats exact match. A heater cycling gently holds temperature after water changes; one running flat-out does not.

Length must fit submerged with minimum water line covered—never turn on dry.

Examples for common UK tanks

40 litres desktop: 50 W in warm room, 75–100 W if above a floor vent. 90 litres bedroom: 100 W minimum, 150 W if north-facing. 125 litres lounge: 150–200 W depending on open top evaporation.

A north-facing bedroom keeper logged their 100 W unit running forty minutes of every hour in January before moving to 150 W—cycle time dropped and the tetras stopped hugging the heater end.

Quarantine tubs lose heat fast—size up or insulate with towels during treatment.

Verifying heater sizing with tests and temperature logs
Verifying heater sizing with tests and temperature logs

Placement and redundancy

Position near filter flow. Keep a backup compact heater for display tanks—set two degrees lower as silent redundancy.

Used marketplace heaters save little and risk stuck thermostats.

Heater positioned for even flow in a community tank
Heater positioned for even flow in a community tank

Verify, do not assume

Dial numbers are starting points. Log temperature weekly for a month after install.

Upsize slightly if unsure—controllers or backup heaters cost less than livestock losses.

Recommended gear

  1. Best overall

    Tetra HT Heater 100W

    Tetra

    Usually £18–£25

    Example 100 W unit for 80–100 litres in average rooms—cold conservatories need the next size up despite the label.

    Best for: 90 litre community tanks in average UK rooms

    Avoid if: Cold conservatory primary heat

  2. Budget pick

    Marina Compact 50W

    Marina

    Usually under £15

    Illustrates nano sizing—will struggle below 18°C room average on anything above 40 litres.

    Best for: 40 litre desks and hospital tanks

    Avoid if: 100 litre main display tank

  3. Also consider

    Interpet Delta Therm Heater 150W

    Interpet

    Usually £22–£30

    Headroom for north-facing bedrooms—long glass tube awkward in shallow tanks.

    Best for: 90–120 litre tanks in cooler rooms

    Avoid if: Shallow nano cubes