A fish tank heater’s use depends on how much of it is automated and what the temperature demands of your fish are. What this means is that most times, you will let your fish tank heater use itself, warming the water to a temperature and then turning it off once the water is warm enough.
But if the fish tank heater is not automated, then you will have to operate it manually. This means that you set it to the temperature you need and then turn it off when it gets there. Many will come with timers to help with that.
You only need to turn the fish tank heater off if your fish prefer a lower temperature. Otherwise, most fish will do fine at the higher temperatures that a fish tank heater can provide.