I think the ones in the photo are the indo type, Megalops Cyprinoides have smaller eyes, longer in length, and angled forhead. i just got a small school of both, nice looking fish, school with dorados!
They grow much bigger than 50cm, I had two that were 90cm, although in a huge tank. They are very, very skittish when confined at a larger size. They also rub their faces quite badly against the glass...Awesome fish though