What size heater do you need? Calculate required wattage based on tank size and temperature difference.
The general rule is 5 watts per gallon for a 10°F difference. For a 10°F difference: 5W/gal. For a 15°F difference: 6-7W/gal. For a 20°F difference: 8W/gal. Always buy slightly oversized — a heater running at 60-70% capacity lasts longer than one running at 100%.