[Chromedome52]

Those are both males, I'd say they're fighting for dominance, not trying to mate.

Easiest way to sex Metynnis species is the anal fin. Males have a round extended area near the front of the anal fin, females have a fairly straight edge on their fin. Not sure exactly which species that is, but the color and markings are probably not a good indicator of sex.

 

[Chromedome52]

Nope, they're fighting. The dominant male is chasing his closest rival, the side by side is showing that he's bigger and stronger. If they were trying to breed they'd be doing barrel rolls, going upside down.

There is one fish I can't get a good look at, and it appears to be larger than the two fighting males. It is possible that the big fish is a female and the two are arguing over who gets to spawn with her, if it is a female.

A moderator should be moving this to the general discussion section sometime soon, shouldn't they? These aren't Cichlids.