Why "watts per gallon" rules often go wrong
A tank loses heat through its glass and water surface, so heat loss follows surface area and the temperature difference between water and room. Small tanks have more surface for their volume than large ones, so they need more watts per litre. A tank in a cold room needs far more than the same tank in a warm one.
Manufacturers say the same thing. Fluval’s E-series volumes assume a 20 °C (68 °F) room and 26 °C (78 °F) water, and the manual warns that a colder room, no lid, or nearby cooling sources may need a higher wattage.
How this heater calculator works
We calibrated the heat-loss coefficient h against published heater charts from Fluval, EHEIM and Aqueon. For every standard US tank in the table below, Aqueon’s chart size falls inside our range. h = 13 adds headroom for cold snaps and rooms that cool overnight. If you only know the volume, we estimate surface area from typical tank proportions (length 2 : depth 1 : height 1.2).
Aquarium heater size chart
| Water volume | Raise 5 °C (9 °F) | Raise 10 °C (18 °F) |
|---|---|---|
| 20 L (5 US gal) | 25–50 W | 50–75 W |
| 40 L (10.6 US gal) | 50 W | 75–100 W |
| 60 L (15.9 US gal) | 50–75 W | 100–150 W |
| 100 L (26.4 US gal) | 75–100 W | 150–200 W |
| 150 L (39.6 US gal) | 100–150 W | 200–250 W |
| 200 L (52.8 US gal) | 150 W, or 2 × 75 W | 250–300 W, or 2 × 150 W |
| 300 L (79.3 US gal) | 150–200 W, or 2 × 100 W | 2 × 150–200 W |
| 400 L (105.7 US gal) | 200–250 W, or 2 × 150 W | 2 × 200–250 W |
Heater size for common US tanks
| Tank | Our range | Aqueon chart (10 °F rise) |
|---|---|---|
| 10 gallon (20 × 10 × 12 in) | 50–75 W | 50 W |
| 20 gallon long (30 × 12 × 12 in) | 75–100 W | 100 W |
| 29 gallon (30 × 12 × 18 in) | 100–150 W | 100 W (30 gal) |
| 55 gallon (48 × 13 × 21 in) | 150–200 W, or 2 × 100 W | 150 W |
| 75 gallon (48 × 18 × 21 in) | 200–250 W, or 2 × 150 W | 200 W |
What heater makers recommend
| Heater | Rated tank volume | Conditions stated |
|---|---|---|
| Fluval E50 (50 W) | up to 60 L (15 US gal) | 20 °C room, water up to 26 °C |
| Fluval E100 (100 W) | up to 120 L (30 US gal) | 20 °C room, water up to 26 °C |
| Fluval E200 (200 W) | up to 246 L (65 US gal) | 20 °C room, water up to 26 °C |
| Fluval E300 (300 W) | up to 379 L (100 US gal) | 20 °C room, water up to 26 °C |
| EHEIM thermocontrol 50 W | 25–60 L | Volume bands only |
| EHEIM thermocontrol 100 W | 100–150 L | Volume bands only |
| EHEIM thermocontrol 200 W | 300–400 L | Volume bands only |
One heater or two?
For tanks over about 200 L (50 US gal), we suggest splitting the wattage across two heaters, one at each end. Heat spreads more evenly, and a failure does less damage: if one heater sticks on, it has only half the power to overheat the tank, and if one fails off, the other still holds some heat while you replace it.
Heater safety tips
- Use a separate thermometer and check it daily. Heater dials are often a degree or two out.
- Do not oversize heavily. A heater much bigger than the tank needs can cook fish quickly if its thermostat fails on. An external temperature controller adds a second safety cut-off.
- Unplug before water changes and follow the manufacturer’s instructions. Many heaters must not run out of water, and a hot heater can crack when it meets cooler water.
- Place it in the flow, near the filter outlet or intake, so warmed water spreads around the tank.
- Fit a heater guard for large or boisterous fish that could break the glass or burn themselves.
- Goldfish and other coldwater fish usually do not need a heater in a heated home.
