Subtract the average temperature of the room the aquarium is located in from the temperature you wish to maintain the aquarium water at.
Using the Aquarium Heater Size Guide below, find the size of your aquarium in the left hand column and move to the column that shows the number of degrees the aquarium needs to be heated. If the heating requirement is between levels, move up to the next larger size.
In larger tanks, or where the room temperature is significantly below the desired water temperature, two heaters may be required. Heaters should be installed at opposite ends of the aquarium to heat it more evenly.
Example
Average Room Temp = 68 degrees F
Desired Water Temp = 77 degrees F
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Heating required = 9 degrees F
Tank Size = 20 gallon
Heater size needed = 50 watts
Aquarium Heater Size Guide
| Tank Size | Heat 5ºC/9ºF | Heat 10ºC/18ºF | Heat 15ºC/27ºF |
| 5 gal/25 L | 25 watt | 50 watt | 75 watt |
| 10 gal/50 L | 50 watt | 75 watt | 75 watt |
| 20 gal/75 L | 50 watt | 75 watt | 150 watt |
| 25 gal/100 L | 75 watt | 100 watt | 200 watt |
| 40 gal/150 L | 100 watt | 150 watt | 300 watt |
| 50 gal/200 L | 150 watt | 200 watt | two 200 watt |
| 65 gal/250 L | 200 watt | 250 watt | two 250 watt |
| 75 gal/300 L | 250 watt | 300 watt | two 300 watt |
