I can assure you, if you held both fish in your hand they would look very different. Not all regions are like this, but many times large adult spottail shiners do not even have a spot on their tail. These are the ones that are a real nightmare to ID compared to eastern silvery minnows which is another closely related species. It's been my experiences that spottail shiners hug the bottom of their habitat. Whereas blacktail shiners and other closely related species like spotfin or satinfin shiners are more of a mid water to surface species.