Aquarium Heater Size Guide

fish swimming in an aquarium tank

Thatsaphon Saengnarongrat / EyeEm / Getty Images

Your aquarium is home to cold-blooded creatures who rely on the temperature of the water to sustain their body temperature. This means that you must provide heat for your aquarium and maintain it at the right temperature for your fish. You can choose from a few different types of heaters and use either one or multiple heaters, depending on your setup. Heater capacity, or heating power, is indicated by its wattage rating.

Types of Aquarium Heaters

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

Hang-on-tank: Hang-on heaters mount onto the rear of the tank (typically) with suction cups and/or a hook or other fastening device over the edge of the tank. They are only partially submerged and tend to be less efficient than other types of heaters, but they can provide adequate heating in smaller tanks. When used in large tanks, it's best to have more than one hang-on heater at opposite sides of the aquarium.

Submersible: Submersible heaters are completely submerged in the tank water and mount to the back of the tank either horizontally or vertically. Often, they are mounted horizontally near the substrate. Submersibles generally provide more consistent and efficient heating than hang-on heaters, especially with larger tanks.

Heating cable: A heating cable is placed under the gravel or substrate and is connected to a control unit. It can be useful for freshwater planted aquariums to eliminate dead spots. However, because you have to dig up the substrate if the cable needs to be repaired or replaced, this type of heater isn't the best option for saltwater reef systems.

Finding the Right Aquarium Heater Size

Sizing a tank heater (or heaters) is based on the water volume of the tank, the average temperature in the room where the tank is located, and the desired water temperature for the tank.

For the heater wattage, the basic rule of thumb is to use between 2.5 and 5 watts per gallon of actual water volume in the aquarium. However, often more wattage is required, depending on how much you want to raise the temperature. If you're using more than one heater, the total heating capacity of all the units together should add up to the required wattage.

To adjust for the room temperature, subtract the average temperature of the room from the target temperature for the aquarium water. The result is the amount of heating required, measured in degrees.

Using the aquarium heater size chart below, find the size of your aquarium in the left-hand column, then 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.


  • Average room temperature: 68 degrees F
  • Target water temperature: 77 degrees F
  • Heating required: 9 degrees F (77 - 68 = 9)
  • Tank size: 20 gallon
  • Heater size needed: 50 watts
Aquarium Heater Size Guide
Tank Size Heat
5 Degrees C
9 Degrees F
10 Degrees C
18 Degrees F
15 Degrees C
27 Degrees F
5 Gallon/20 Liter 25 watt 50 watt 75 watt
10 Gallon/40 Liter 50 watt 75 watt 75 watt
20 Gallon/75 Liter 50 watt 75 watt 150 watt
25 Gallon/100 Liter 75 watt 100 watt 200 watt
40 Gallon/150 Liter 100 watt 150 watt 300 watt
50 Gallon/200 Liter 150 watt 200 watt two 200 watt
65 Gallon/250 Liter 200 watt 250 watt two 250 watt
75 Gallon/300 Liter 250 watt 300 watt two 300 watt

The Spruce / Hilary Alison

Tips for Using Aquarium Heaters

  • In larger tanks, or in situations 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 ensure even heating.
  • It is best to use multiple units with hang-on and submersible heaters. This provides more even heat distribution and puts less strain on the heaters. Also, if one of the heaters 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. As a general rule, the heater tube should match the height of your aquarium.
  • Check for heat sources and fluctuations in room temperature around the aquarium. Your tank may be located under an air vent or next to intermittent heat sources that can make the temperature rise and fall.
  • Always unplug the heater when you are draining the tank to prevent the heater from overheating when it is no longer submerged.
  • Some heaters have specific number settings for the thermostat control so you can set it to the desired temperature. Other heaters have controls that go from low to high, without having specific temperatures indicated. The former ones are easier to use, but always check to ensure the heater setting is actually keeping the aquarium at the correct temperature.
  • Have the heater located by the water outflow from the filter so that the moving water disperses the heated water throughout the aquarium.
  • Use an aquarium thermometer to verify the heater is keeping the aquarium water at the correct temperature. Move the thermometer around to check the temperature in multiple locations in the aquarium to ensure that the water temperature is uniform.