This is a discussion on Male & Female? within the Characins forums, part of the Freshwater and Tropical Fish category; -->
The difference in coloration on the fins of these Bloodfin Tetras make me think it could be a male and female. Anyone know? Note ...
The true Bloodfin, Aphyocharax anisitsi, has identical colouring in male and female. The male has a small hook on the anal fin. This fish on the right in your photo is this species.
A very similar fish, the Glass Bloodfin, Prionobrama filigera, has the appearance of the fish on the left in your photo. In this species, male and female are also the same, having a red caudal fin (quite bright red). The male's anal fin is extended and has a small spot of black behind the white stripe.
Both species are about the same adult size, and being shoaling fish should be kept in a group of 6 or more. Water parameters are basically identical for both species.