Cupid Cichild

I want to buy this fish for my 172 gal. Tank.
I've been reading about them. In the pet shop We can to find the Biotodoma
Cupido "Santarem" (Cupid Cichild) fish from Brazil, harmless and don't have
problem with small fishes and the Biotodoma Cupido sp. from West-Guyana,
agressive and very dangerous for de small fishes.

I've been watching a lot photographs both of them, and Icouldn't tell them
apart.

Could anyone tell me any external difference to teel apart in the pet shop

Thanks in advance