Aquarium Heater Size Guide

aquarium heater size

Illustration: © The Spruce, 2018

Your aquarium is home to cold-blooded creatures who rely on the temperature of the water to sustain their body temperature. You will need to provide heat for your aquarium and maintain it at the right temperature for your fish.

Types of Aquarium Heaters

You have a choice of the type of aquarium heater you choose and how many to use.

  • Hang-on-Tank Heater: This type is only partially submerged and is less efficient but may provide basic heating.
  • Submersible Heater: You will get more consistent and efficient heating with a submersible heater.
  • Heating Cable System: This system is placed under the gravel or substrate and connected to a controlling unit. It can be useful for freshwater planted aquariums to eliminate dead spots. Because you would need to dig up your substrate if it needs to be repaired or replaced, it isn't good to use for saltwater reef systems.

Finding the Right Aquarium Heater Size

The basic rule of thumb for wattage is to use between 2.5 and 5 watts per gallon of actual water volume in the aquarium. You can use a single unit of the correct size or multiple units that add up to the needed wattage. But then you need to adjust for the temperature of the room and the temperature desired in the tank.

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
5 Degrees C
9 Degrees F
10 Degrees C
18 Degrees F
15 Degrees C
27 Degrees F
5 Gallon/25 Liter25 watt50 watt75 watt
10 Gallon/50 Liter50 watt75 watt75 watt
20 Gallon/75 Liter50 watt75 watt150 watt
25 Gallon/100 Liter75 watt100 watt200 watt
40 Gallon/150 Liter100 watt150 watt300 watt
50 Gallon/200 Liter150 watt200 watttwo 200 watt
65 Gallon/250 Liter200 watt250 watttwo 250 watt
75 Gallon/300 Liter250 watt300 watttwo 300 watt

Heater Selection Factors

  • It is wise to use multiple units with the hang-ons and submersibles. It provides a more even distribution of heat and doesn't strain the heaters. If one goes out, the temperature may not plummet too dangerously until you can get a new unit. It's also smart to buy an extra heater to keep on hand as a backup.
  • The heater tube length is important because heat rises. It should match the height of your aquarium.
  • Check for heat sources and fluctuations. Your tank may be located under a vent or next to intermittent heat sources that can make the temperature rise and fall.
  • Be sure to unplug the heater when you are draining the tank. Otherwise, it may overheat when it is no longer submerged.