Sizing an aquarium heater
The standard guideline is 1 watt per litre for tanks in a room that rarely drops below 18°C. In colder rooms or for large tanks, size up. This page explains the practical rules and the failure modes to avoid.
Wattage reference
These figures assume a target temperature of 25–26°C and a room that stays above 18°C. If your room drops below 15°C in winter, add 20–30% to the wattage. For tanks over 200L, use two heaters of equal wattage (one each end) rather than one large heater — this provides redundancy and even heat distribution.
| Tank volume | Warm room (>18°C) | Cold room (<15°C) |
|---|---|---|
| 30L | 50–75W | 75–100W |
| 60L | 75–100W | 100–150W |
| 100L | 100–150W | 150–200W |
| 200L | 200–250W (or 2×100W) | 250–300W (or 2×150W) |
| 300L | 300W (or 2×150W) | 2×200W |
These are guidance ranges. Actual heating requirements vary by tank material, depth, and room conditions. A thermometer is your final check.
Visual: heater wattage midpoints
Chart values are midpoints of the warm vs cold room bands in the table — for comparison only, not a product rating.
Placement matters
Place the heater near a point of water flow (filter output or circulation pump) so warm water distributes evenly. Horizontal placement near the bottom works well in tall tanks. Avoid placing directly against substrate — it traps heat.
Stick-on thermometers are not enough
Liquid crystal strip thermometers measure glass temperature, not water. Use a digital probe thermometer placed at the far end from the heater to verify actual water temperature.
Heater failure risks
- Heater stuck on (cooking). A heater that fails in the "on" position will overheat the tank rapidly — a common cause of total livestock loss. Dual heaters with a controller reduce this risk. Check the heater periodically even if the tank seems fine.
- Heater removed briefly — thermal shock. Never operate a glass heater outside water; it can crack when submerged again while hot.
- Thermostat calibration drift. Heaters over 2–3 years old may read inaccurately. Verify with an independent thermometer after seasonal temperature changes.
- Using a heater in a very small tank without flow. In a 10–20L tank with no circulation, the water directly around the heater can be 2–3°C hotter than the rest of the tank.
- No redundancy in species that require heat. For tropical fish, consider a backup heater in storage during winter, especially for discus, altum angels, or other heat-sensitive species.
Related tools and pages
- Tank builder — check temperature overlap across your whole stocking list.
- Filter guide · Tank setup guide · All setup guides
- Browse by tank size: 30L · 60L · 100L · 200L