When you see alot of payara, it's a natural feeling, 100% armatus.
First the color of the apidose fin in contrast with the tail fin, second the body shape and third the color.
never owned armatus of this size but can say it doesnt look like a scomb or tat of that size. the lack of red color (not a tat) but could be camera issue. the shape of the body and tail rules it out as scomb. curve of the back forward of dorsal fin to nose is different to a tat as well.
which shop did you get it from and if dont mind telling, price? Armatus have been coming through town but have not seen one in this size.
worth trying with the tat, as pretty low risk to armatus. just feed it the common feeders you find at the corner shop. there's one in the photo above. "leng yu" or whatever comes in that size. It can eat those fan tail gold fish and baby koi as well.