could it be one of black neons is very plump could she be pregnant unfortunatly i did notce until i got them from a friend and di not notice till i got home my friends tank is not realy well maintained could she
They can be bred in the aquarium, though it is somewhat more tricky than most. I would doubt the chances of her being pregnant are sort of slim, but you never know. They are egg scatterers so keep watching for those eggs.
I've never tried to breed and collect eggs from any Tetras, but the female is often larger in the belly than the male in many fish species. Female egg layers and scatterers can fill with eggs reabsorb them or scatter them whether there is a male or not. Thing is eggs and fry in the aquarium will most often be eaten unless the parents are removed or the eggs are harvested and cared for by the hobbyist.