Sorry for getting back to you so late. The scientific name of your payara is Hydrolycus tatauaia. One of the most distinguishable characteristics is the golden body with red fins. However, there are some that don't have this coloring. I've owned one in the past that was all grey with grey and black fins. There are different varients of payara, just like any other fish, depending on location and the water chemistry of those places. Hydrolycus scomb. has a round black spot near the gill cover, and it also has black spots at the base of the pectoral fins. These are the ones that usually die off at around 12". H. tatauia and armatus have more of a moon shaped black blotch near the gill cover. The also have different body structures. They may not stand out as much seeing them separately, but once they are next to one another the differences are quite obvious. Tats and armatus have a much more developed jaw and they don tend to swim as diagonally as scombs. Armatus have a bright orange adipost fin, a white band at the edge of their tail, an even more developed jaw, and thay can reach 2' in an aquarium. Both Tats and armatus are a lot more active and aggressive than scombs (which usually just sit there and tick in the same spot). If you have the money and can even find one, Armatus are the way to go. However, redtails are also a kickass fish are much cheaper and more readily available. Sorry for all the typoes, but I wrote this pretty quickly because I'm at work. Congrats on your new pick up.