It is the temperature gap that decides
A heater's job is to bridge the gap between the coldest your room gets and your target water temperature—usually 24–27°C for tropical fish. Measure the room at its coldest (winter night, heating off), not its average. For a lift of up to 5°C, roughly 0.75 W per litre suffices; 5–10°C wants about 1 W/L; more than 10°C calls for 1.25 W/L or splitting across two units. Our heater calculator applies exactly this logic and rounds to real product sizes.
Quick reference chart
For a typical 5–8°C lift: 20–40 L → 25–50 W; 60 L → 50–75 W; 100 L → 100 W; 150 L → 150 W; 200 L → 200 W or 2 × 100 W; 300 L → 2 × 150 W; 400 L → 2 × 200 W. If your room regularly drops below 17°C, step one size up. If your home never falls below the target water temperature, a small heater simply as a thermostat safeguard is enough.
Two heaters beat one on big tanks
From roughly 200 litres, splitting wattage across two heaters at opposite ends brings three benefits: more even heat distribution, redundancy if one fails off, and slower overheating if one sticks on—the most dangerous common failure. An external thermostat controller (~$15–25) adds an independent cut-off and is arguably the best value safety upgrade in the hobby. Always run a separate thermometer; heater dials are frequently off by 1–2°C.
Placement and habits that make any heater work better
Mount the heater near the filter outlet or in the strongest flow so heat distributes instead of layering; horizontal mounting near the substrate also works well since heat rises. Unplug it during water changes—an element exposed to air for a minute can crack the tube when cold water returns. Give a new heater 24 hours with a thermometer before trusting it with livestock, and recheck the calibration seasonally.