[hoppycalif]

With the room temperature at 78F you don't need a heater at all. It could only warm up the water more, not cool it down. I don't use a heater in my 45 gallon tank, because the room rarely ever gets as cold as 65F, and only in the winter. That size tank cools down pretty slowly, so if the room is warmed up every morning back to 78F it probably will never be too cool.