All animals and plants have evolved under specific environmental conditions. Some can survive under a wide range of conditions and others have a very narrow tolerance range. Organisms with a wide tolerance of environmental conditions are often common and widespread. Humans, dogs, cats, rats, mice, cockroaches and pigeons are all examples of animals with wide environmental tolerances. They are all nearly ubiquitous around the world. Most organisms require a much narrower range of environmental conditions.
Bettas actually have a wide temperature tolerance range, but only its lower end coincides with the upper end of the typical human comfort zone. Bettas tolerate from the low 70’s°F to the upper 90’s°F. Most people, especially in North America and Europe are comfortable between 65°F and 75°F, with cooler temperatures at night. This means that in our homes, the overlap range is 70°to 75°F. Unless you keep your house warm, you should have a heater in your Betta tank.
If you don’t want to heat your aquarium, there are many fish that you can keep. Zebra danios, cherry barbs, paradise fish, many killifish, many North American native fish, many Goodeids, goldfish and Gymnogeophagus cichlids are among the more common cool water aquarium fish. These are all comfortable at 65°to 70°F. Only the Paradise Fish, Macropodus opercularis, is appropriate for an aquarium without filtration.