DarkFin

I have looked at photos and everything, but i cant understand how to sex them...

TFA101

if you have sailfin moolies, the males have a larger dorsal than the females

but the most reliable way to find out which is male and female is by looking at the anal fin, which is just behind their belly. The male's fin is modified, a rod-like fin, called a gonopodium, they use to insert into the females vent when mating. The females have a fan shaped anal fin, just like a typical fish. Check out the link below to see a comparison illustration.

http://thesmileyfish.com/wp-content/...calfish010.jpg

OldMan

The only reliable way to sex common livebearers is by the shape of the anal fin. In a common male it is called a gonopodium and is shaped like this one on a Heterandria formosa male.



The same anal fin on a female is this more or less triangular shape.



The anal fin is the one with a spot on it on the bottom of the fish in the second picture.