Heaters
From WikiAquarium
Heaters come in different sizes that are measured in watts. Multiply the number of gallons in your aquarium by 5 to determine the size heater to get for your aquarium.
For example, a ten gallon aquarium needs a 10 x 5 = 50 watt heater. Similarly a 20-gallon aquarium needs a 100 watt heater.
When using an aquarium heater always have a thermometer available as well.
Cool water aquariums do not require a heater, but a thermometer is always useful.
