In my experience you can tell the difference even when small by the width the of the mouth, Sanitwongsei (paroon) have a much wider mouth (full width of the head), they have shark like fins ie. Pointed, and they "never" stop swimming, constantly on the move at speed. I found them to be faster growing than any iridescent shark (usually just labled pengasius in the uk). Iridescent sharks tend to be slower swimming, more cruising around, although this is easier to tell difference when larger. In the UK the paroon is sold as the high fin pengasius.