Aptly named for their red fin color, the differences between the sexes of bloodfin tetras are few and far between. You often won't know that you have a female swimming around until she sends eggs to the bottom of the tank. And you may not see the eggs at all if you have a few males wandering about.
Look at the size of your little swimmers. Female bloodfin tetras don't mind showing off their body, which is a bit rounder than the males, even if the female isn't pregnant. This is one of the easiest differences to spot between the sexes.
Inspect the anal and pelvic fins for a white tip, indicating a male bloodfin tetra. The pelvic fin is located closest to the head on the underside of your tetra's body. It's not the same as the pectoral fin, which is located closer to the head but on the side of the fish. The anal fin is the fin closest to the tail on the underside of the fish. The white tips can be difficult to see, so make sure you look at the fish in bright light.
Look at each tetra's color. Males one-up the females by showing off brighter and more dramatic colors, but the difference is minimal. Good light conditions make the colors of the male more distinct, but for maximum effect, you'll probably need to keep your face nearly pressed up against the glass to tell.
Take a picture of your tetras when they're near the front of the tank. Look at the shape of their anal fin. If a sliver of the top portion of the anal fin appears disconnected and takes on a hook-like appearance, you have a male bloodfin tetra. The hook shape is incredibly difficult to spot on moving fish, so it's easier to just take a picture.
Pay attention to the tetra's behavior if a female lays eggs. Males see the eggs as fish pellets and will eat them in a hurry. Females will do the same, but generally not at the same rate as the males.
- Sneak a peek at your tetras during the day time. Avoid shining a flash light to inspect them at night, because the sudden burst of light will stun and disorient them.