This is a huge hole in my knowledge. IDK how to tell any pima species apart. Now there are several more species, according to the latest, recent revision.
I've read somewhere that we do get a mix in the trade of gigas and arapaima arapaimas (they come from farms). I read it long ago and forgot much. Perhaps Rod of Predatory Fins can shed some light or Steve of Exotic Fish Shop. I mean vendors who bring them in may know something, if they have a curious mind... Our two came from Steve back in 2016.