Aquarium Heater Size Guide: Watts Per Gallon Calculation

Temperature stability is critical for the health of your aquatic pets. In the wild, bodies of water change temperature very slowly. In a small glass box, however, temperature swings can happen rapidly, causing stress, weakened immune systems, and diseases like Ich.

Calculating Heater Size: The 5-Watts-Per-Gallon Rule

To maintain a stable tropical temperature (usually 78°F / 25°C) in a standard home environment, the general rule of thumb is 5 Watts of power for every 1 Gallon of water volume.

Not sure of your tank size? Check our Calculator Tool first.

Aquarium Heater Placement for Consistent Temperature

You can buy the best heater in the world, but if you put it in a "dead spot" with no water flow, it won't work correctly. The heater should always be placed near the filter output or in the stream of a circulation pump. This ensures the heated water is quickly distributed throughout the entire tank, rather than creating a hot spot around the heater itself.

Also, keep in mind that certain fish types mentioned in our Stocking Guide require higher temperatures (like Discus), which may require a higher wattage per gallon ratio.