The 5W-per-gallon rule (and where it breaks)
The standard hobby rule of thumb is 5 watts per gallon for a 5–10°F lift over typical room temperature in an insulated home. The rule is approximate — heat loss is a surface-area phenomenon and surface-to-volume drops as tanks grow, so very large tanks don't scale linearly. Very small tanks (under ~5 gallons) also break the rule because the smallest commonly-available heaters are 25W.
The calculator above adjusts watts-per-gallon by the actual temperature delta you specify:
- ≤ 5°F lift: ~3 W/gal
- ≤ 10°F lift: ~5 W/gal
- ≤ 15°F lift: ~7.5 W/gal
- > 15°F lift: ~10 W/gal — and consider room insulation
Why two heaters on big tanks
For tanks over 75 gallons, conservative practice is two heaters at half the recommended total wattage each. Two failure modes motivate this:
- Stuck-on heater. A failed thermostat that keeps the heater always-on cooks the tank. Two smaller heaters limit how high a single failure can drive the temperature.
- Stuck-off heater. A failed heater during a cold-night power-cycle drops the tank fast. A second still-working heater keeps the tank above the danger floor.
Target temperatures by common species
- Goldfish: 65–72°F (cool-water; usually no heater in temperate rooms)
- Most community tropicals (tetras, corydoras, guppies): 76–78°F
- Bettas: 78–80°F
- Discus: 82–86°F (high delta — requires more wattage and stable room temp)
Related tools
- Tank volume calculator — get accurate water volume first.