most deffinetly a serr. of some sort. Looks young still and hard to tell as he will grow into his true colors. I agree with big roc looks like a black rhom. but could be a sanchezi or something like it. sorry i wasen't more help.
Definably a Serrasalmus species, but at the age it could be nearly any in the category.
Just keep him/her in a roomy tank, on its own, and fed well. The sooner it starts to grow the sooner we can tell.
the pics are crap even out of water, try getting them in focus.
anyway probably a rhom but needs to grow as advised.
as for hollandi valid name but no valid species attached to it.