Are these really black monk tetras?

[TheSwissAquarist]

Hi all; I recently bought 6 of these guys (they were flaring in the store, I couldn’t help myself!) and was wondering if 2 of them are actually a different species? 4 are a sort of deep black, while the other 2 have red ventral fins.

Help appreciated! I’m trying to get into Tetra breeding for my clubs annual auction. 

[TOtrees]

Hi! So common names are really problematic. I don't recognize the name you're asking about, "black monk tetra" (can't find it online either). But the fish in your pics look like what we over here know as black phantom tetra. Seriously Fish has this to say about sexual dimorphism in this species: "Adult males possess black fins of which the pelvic, dorsal, and anal are enlarged, while females are deeper-bodied and tend to have reddish pelvic, anal, and adipose fins." This seems consistent with your first pic being males, and your second being a female. 

[TheSwissAquarist]

Thanks so much!