Just to add... Your tank is unlikely to be the depth or size of a river or lake, therefore will cool down / heat up more quickly than either of those would. Further, your fish are going to be kept in a ore unnatural environment - at the end of the day, it will always be a glass box, with higher stocking levels, and my fish are too stupid to know there are no predators in their glass box - so likely unavoidably more stressfull to start with.
HOWEVER, if the room your tank will be in fluctuates no more than a couple of degrees and stays in the temp range for the species (unlikely), I'd say, don't get a heater.