The best method to work out the wattage of heater required for your home aquarium fish tank is to consider the average ambient temperature of the room and work out how much extra heat will be required to increase the temperature of the water above this.
| Tank Size | Heat 5ºC/9ºF | Heat 10ºC/18ºF | Heat 15ºC/27ºF |
| 5 gal/25 L | 25 watt | 50 watt | 75 watt |
| 10 gal/50 L | 50 watt | 75 watt | 75 watt |
| 20 gal/75 L | 50 watt | 75 watt | 150 watt |
| 25 gal/100 L | 75 watt | 100 watt | 200 watt |
| 40 gal/150 L | 100 watt | 150 watt | 300 watt |
| 50 gal/200 L | 150 watt | 200 watt | two 200 watt |
| 65 gal/250 L | 200 watt | 250 watt | two 250 watt |
| 75 gal/300 L | 250 watt | 300 watt | two 300 watt |
I would always recommend using 2 heaters for your aquarium which are equal to the total wattage required. So for example if you have a 100 litre aquarium and need to heat it from 18 degrees to 28 degrees (10 degree C increase) then the table recommends using a 100W heater. I would however use 2x 100W heater. Provided you are using thermostat integrated heaters (of which most are these days) then you will not use up any more power or electricity as they wont be on for as long to heat the water. This is simply a precautionary measure in case one of the heaters fails, the secondary heater would still be capable of heating to the correct temperature, giving you time to purchase a replacement second heater.
This is purely a guide and should give a good rough estimate of what you would need for your fish tank to ensure the right temperature is maintained. It cannot hurt to have too much wattage of heat in the aquarium as they will simply turn off if the tank has reached the desired temperature.
2 Heaters better than 1!