Aquarium Heater Size Guide

Fish aquarium
Pankaj Danidhariya / EyeEm/Getty Images


First, subtract the average temperature of the room the aquarium is located in from the temperature at which you wish to maintain the aquarium water.

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 a situation 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.


Average Room Temp = 68 degrees F

Desired Water Temp = 77 degrees F
Heating required = 9 degrees F

Tank Size = 20 gallon
Heater size needed = 50 watts

Aquarium Heater Size Guide

Tank SizeHeat 5ºC/9ºFHeat 10ºC/18ºFHeat 15ºC/27ºF
5 gal/25 L25 watt50 watt75 watt
10 gal/50 L50 watt75 watt75 watt
20 gal/75 L50 watt75 watt150 watt
25 gal/100 L75 watt100 watt200 watt
40 gal/150 L100 watt150 watt300 watt
50 gal/200 L150 watt200 watttwo 200 watt
65 gal/250 L200 watt250 watttwo 250 watt
75 gal/300 L250 watt300 watttwo 300 watt