I bought a bleeding heart tetra, and this little fella ages ago, not sure what he is though! He's always flaring up in black and deep red and wqould like to get him some playmates! [He's the middle one in the first pic and at top right in the other.
Agree with Colin_T and also a very good example of a Platty that's ready to pop within the next (at least) 6 - 8 Hours.... approximately 35 to 42 fry in there if you can save them all.
given the lack of white tips on the dorsal and ventral fins, I'd say it is a roberts' tetra Hyphessobrycon robertsi.
there is a profile for them in the fish profile section of this forum