This is most likely due to their small size, bright coloration and schooling personality.
Schooling Fish are Recommended - Good examples of schooling fish include most Tetras and dwarf varieties of Australian Rainbow Fish. Threadfin Rainbowfish, Praecox Rainbowfish and other dwarf varieties will school in larger groups in a suitably sized aquarium. Ember Tetra's have also gained in popularity. This is most likely due to their small size, bright coloration and schooling personality.