That fish is absolutly NOT a Rhom!!! I have kept many Black Rhom's in my career and that is without a doubt not a Black Rhom. I agree that it is either a serr. Manuelli or a Eigenmanni. They both have the elongated vertical stripes on their sides, a blunt snout and a dark black spot behind the gills. Although, this is a great fish i have kept two . . . wow and one of the most colorfull piranha ever..... unfortunately one got eaten in half by two black rhoms a gold and three red's. It sickened me to loose that fish. What gives this fish away as a smart and dangerous hunter is the size of its eyes. They have a massive field of vision with those things nothing like any other piranha out there. Cool fish man but most definitlely not a black rhom.
