Is your red fin tetra male or female? There are ways to determine the gender of these marine animals and they will be discussed here. It is usually easier to distinguish a female fish from a male if they are placed side by side. The male will look slimmer than the female. When a female is ready for spawning, her belly becomes very fat.
You can also tell male characins differently from female red fins by looking at their coloring. The blue stripe that is present in both genders looks straighter in the water on a male. By using these two tips, you can easily select two or more fish for mating. Breeding tetras is discussed in another article on this website.
When the males are fully mature, their fins are more developed than their counterparts. characin males can really be distinguished easily by using the fact that their dorsal fin is more elongated. Female tetras have a more rounded dorsal fin and after you have looked at a few of them, you will identify them quickly this way.