Instructions:

Subtract the average temperature of the room the aquarium is located in from the temperature you wish to maintain the aquarium water at. Find the size of your aquarium in the left hand table 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 heater.

In larger aquariums or where the room temperature is significantly below the desired water temperature, two heaters may be required. If this is the case, install the heaters at opposite ends of the aquarium to heat it more evenly.

Heater size guide

Gallons/litres	5oC/9oF	10oC/18oF	15oC/27oF
5 gal/25L	25 watt	50 watt	75 watt
10 gal/50L	50 watt	75 watt	75 watt
20 gal/75L	50 watt	75 watt	150 watt
25 gal/100L	75 watt	100 watt	200 watt
40 gal/150L	100 watt	150 watt	300 watt
50 gal/200L	150 watt	200 watt	2x200 watt
65 gal/250L	200 watt	250 watt	2x250 watt
75 gal/300L	250 watt	300 watt	2x300 watt

Example

 
Average room temperature = 15 degrees C

Desired water temperature = 25 degrees C

Heating required = 10 degrees C

Tank size = 100 litres

Heater size = 100 watts