I believe you are talking about the Black Skirt tetra.
The female Black Skirt tetra is usually plumper than the male one. Also, the female's anal fin runs parallel to the second vertical line on her abdomen while for males, it slants back towards the tail. Moreover, the anal fin of the female is narrower than the male anal fin.The dorsal fin is also less pointed in females.
The female Black Skirt tetra is usually plumper than the male one. Also, the female's anal fin runs parallel to the second vertical line on her abdomen while for males, it slants back towards the tail. Moreover, the anal fin of the female is narrower than the male anal fin.The dorsal fin is also less pointed in females.
