What Size Heater is Needed for an Aquarium?

Aquarium Size

• Calculating simply by volume, a 5 gallon (19 liter) aquarium will need no more than 25 watts to heat, and a 50 gallon (190 liter) aquarium would require 250 watts. Larger aquaria, with more water to heat, will naturally require larger heating elements; more heat is achieved through more watts of power. Most manufacturers of aquarium heaters list the size of aquarium for which each heater is designed. For an extra large aquarium, 100 gallons (378 liters) or larger, using two smaller heating elements that add up to the total wattage needed may be more effective than one large heater, especially if one heater fails. A drop in temperature even two degrees can be devastating to the health of tropical fish and other tropical aquarium life. In the 100 gallon example, you would use two 250 watt heaters placed at opposite ends of the tank.

  Fish can overheat if water temperatures rise above 78 degrees Fahrenheit or 80 degrees Fahrenheit (26 degrees Celsius). Aquarium heaters are fitted with internal thermostats that prevent overheating when set properly, but they cannot cool water once it is too hot. If you live in an especially warm climate, where the average temperature is above tropical water temperatures, you will need to consider cooling as well.

Ambient Temperature

• A more complex measure of heater size required involves calculation of the mean temperature difference between the ambient (room) temperature and desired water temperature. Measure the average temperature in the room where the aquarium is kept. Most people keep their house at an average of 68 degrees Fahrenheit (20 degrees Celsius). Tropical fish typically require temperatures of 75 degrees Fahrenheit to 77 degrees Fahrenheit (25 degrees Celsius) to remain healthy. Therefore, your aquarium heating element will need to raise the water temperature about 9 degrees to reach the proper temperature. Your aquarium lighting may be generating heat as well, and any heating or cooling equipment in your home situated near the aquarium can affect the water temperature.

  For the average room, you will want to raise the water temperature from 68 degrees Fahrenheit to 77 degrees Fahrenheit. This is a net change of 9 degrees Fahrenheit (5 degrees Celsius). Some manufacturers recommend different wattages per gallon based on the net change of temperature needed, broken down as 9 F (5 C), 18 F (15 C) and 27 F (15C). For example, a 10 gallon (50 liter) tank would need a 50 watt heater (5 watts per gallon) to raise the temperature 9 degrees F (5 C), but requires 75 watts to raise the temperature from 50 or 60 degrees F (10 to 15 C) to 77 F (25 C).