Nope! They actually don't need a heater. If you have an average room temperature they will do fine!
None of my 60 tanks overhere have heaters. A lot of livebearers like guppies will also do well at lower temps. I even keep them every year from spring till mid fall in unheated outdoor tanks.
If the tank is overcrowded or overfed, a filter would do the job. But I do have to admit that I do have some tanks without a mechanical filtration. Just a natural balance and filtration is doing the job. But I do have to admit that it needs a bit more experience to keep this wellbalanced. But it works!