Ah, now I understand why no picture was showing before. Doesn't show up, unless you log in.
This is a really tough one to identify, IMO, considering how many Crenicichla species it could resemble at various stages in their life. Could even be some type of dwarf pike, though I'm still thinking it's a juvie.
I'm going to assume it's a little older then I figured on my first guess , but still a juvie. Crenicichla saxatilis?