I understand, and I am the same way. When I first got it, that was what I had thought as well because of the tail fin. While I was looking and gathering info, The yellow ray had pics also with tail fins, so I started looking closer.
http://en.wikipedia.org/wiki/File:Yellow_stingray_mote_marine.jpg
They are very close look alikes in the pics, but what made me believe it was the yellow, is on the round rays wiki page, the drawing on the left side shows the rear fins on the disk as having those "streamers" that are like at 45 degree angles from the tail, and the round ray also says it has a short thick tail with a "large stout" stinging spine, showing the spine close to the base of the tail.
The yellow description says it has a leaf shaped tail fin that is 1/4 high as it is long, and completely covers the last vertebrae, and has a small stinging spine in the middle of the tail.
They are very close in description though. If it turns out to be a round, and shows any signs of stress, I will make sure it gets into the proper environment for it.