while the temp (air) may fluctuate the same is not true with the water.
the air may fluctuate 10,15 even 20 degrees but the water will not dissipate heat as quickly. large bodies of water may not show any significant change in temp.
the problem here is the aquarium.. it has at least 5 exposed sides (in most cases) this larger surface area in addition to the relatively small water volume..will fluctuate.
if the change is only a few degrees over several hours the tank inhabitant s may be fine.
I would opt for a heater it will help to stabilize water temp.
btw.. some species that require warmer water..ie Discus and Blue rams.. will need a heater as temps only maxing at 79 or so is too cool for them in the long run,(IME)