A few smaller ones.
In some cases it isn't possible to heat a tank with one heater.
1 watt per litre. More if it's in a colder climate 2 if it's particularly cold.
120 us gal
120 imp gal
What's a Canadian gallon?
A pair of 250w-300w, some fish like it warmer so you may need more.
If the air temperature is considerably different you will need more energy to heat the water to the same temperature.