What size heater do I need for my aquarium?  The answer to this question is not as simple as it sounds.

First you need to know a few details about the aquarium;

• What volume of water does your aquarium hold? (Calculate)
• What is the average Winter temperature in the aquariums location?
• What water temperature is required in your aquarium?

Now that you have this information you can to calculate the maximum temperature increase required for your tank.

For Example:
Average winter temperature = 12ºC 
Water temperature required = 22ºC
22ºC-12ºC = 10ºC temperature increase needed 

Now that you know the maximum temperature increase required, use the chart below to calculate your heater size.  Find the temprature increase needed in the top row, move down this column untill you reach your tank size (shown in the left column).

For Example:
Temperature increase needed = 10ºC
Tank volume = 100L
The chart shows that we require a 100watt heater.

Liters/Gallons (std tank size)	5ºC/9ºF	10ºC/18ºF	15ºC/27ºF
25 L/5 gal (1ft)	25 watt	50 watt	75 watt
50 L/10 gal (2ft)	50 watt	75 watt	75 watt
75 L/20 gal (3ft)	50 watt	75 watt	150 watt
100 L/30 gal (4ft)	75 watt	100 watt	200 watt
150 L/40 gal (6ft)	100 watt	150 watt	300 watt
200 L/50 gal(8ft)	150 watt	200 watt	two 200 watt
250 L/65 gal (10ft)	200 watt	250 watt	two 250 watt
300 L/75 gal	250 watt	300 watt	two 300 watt

NOTE: Standard tank sizes are 1ft (30cm) wide by 1ft(30cm) deep by the length.  Use Aquarium Monsters Volume Calculator for rectangular aquariums to find the exact volume of your tank.

Tips and Tricks:

If in doubt always use a slightly larger heater than required, this will limit the amount of time your heater is running and allow it to increase water temperature faster.

In larger tanks or tanks there the ambient temperature is significantly below the required water temperature the use of 2 heaters placed at opposite ends of the tank and/or a pump to circulate water, helps with even heating of the aquarium and stops temperature variations developing.

The use of an aquarium heater with an external thermostat is highly recommended, as it will allow fine tuning of water temprature and allow for easy adjustments if needed (EG: treating sick fish).