The right size of aquarium heater depends mostly on the water volume in the tank and the steady temperature that you want to keep. Bigger water mass needs more heating surface, so the tank with a lot of water needs a heater of higher power than small aquariums. For example, for 10 gallon aquarium you need a 50 watt heater, while 25 watt is enough for 5 gallons.
A 14 gallon tank wants a 75 watt heater based on the rule of 5W for gallon, but 100 watt also works.
How to Choose the Right Aquarium Heater
The temperature change between the aquarium and the room also affects the right size. For example, if the room is 5 degrees cooler than the water, you must use a 30 watt heater. A general rule of thumb says 5 watts for 1 gallon, when you heat 10 degrees above room temperature and use a cover to save the heat.
For 10 degrees change you need 25 watts for 10 gallons, so for a 40 gallon aquarium you need four 25 watt heaters. Some makers advise 5W for gallon for 5°F above room, 10W for 10-15°F above room and 15W or more for gallon for 15°F or more above room.
For big aquariums or when the room temperature is a lot below the wanted water temperature, it is good to use two heaters. Above 75 gallons two heaters help against failure. For a 50 gallon tank you use 200 watts, and for a 200 gallon three 200 watt heaters are used.
Two full sized heaters with a control is a good option. If one breaks, the second keeps the temperature. For safe heating you install them at opposite sides of the aquarium.
It is more good to lay two or three little heaters than one big, because the water flow is usually flat, so you incline them in 45 degrees instead of lay flat.
Too big heaters risk to cook the fishes, if they strangely overheat. Sometimes more good to choose a little but good aquarium heater. Usually you rather take the most little size that does the job.
If the involved power falls between two levels, you choose the next bigger.
