A Water Heater is necessary if your temperature drops below 60 F consistently. Heaters should be set to the lowest setting and adjusted minimally once per day in small increments until the desired temperature is achieved. Note that jellyfish don't necessarily like warm water, but this heater is also useful to maintain temperature stable in the event that there is constant or daily fluctuations in temperature.
NOTE: You should purchase 2 separate heaters - 1 to be used to regulate the aquarium temperature and the 2nd to keep the new water for water changes set to the same regulated temperature. Preforming a water change with water that is a different temperature than the aquarium can injure the animals.
Pro tip: A heater can also be used during the cycling process to speed up the cycling process.