Bit of a long shot, but from the deep body shape, perhaps a Colombian Tetra?
I've still yet to deal with my unwanted "Brucy bonuses" when I bought a group of social Pyjama Synodontis about a month a go... During that time, two Convict Cichlid fry have at least tripled in size to ~2cm TL, which although they will love my hard water could wreak havok as they get older!