What size of heater is best for your aquarium?
Selecting the right heater is critical
For heating requirements, Carl Strohmeyer, Aquarium Answers, recommends 25 watts of heater for every 10 degrees of ambient temperature per 10 gallons you need to raise your aquarium temperature.
EXAMPLE: If your home is 68 degrees and you have a 40 gallon aquarium, to reach a temperature of 78 degrees you would need a 100 watt heater.
How is this worked out - the difference in temperature is 10 degrees, which requires 25 watts of heater to raise the temperature of 10 gallons. As you have a 40 gallon aquarium you need 4 heaters (4x10gallons) at 25 watts to heat the water. So a 100 watt heater is ideal.
The following table assumes 25 watts are required to heat 10 gallons by 10 degrees: