For information on sexing fish that I have kept, see the list of species on my fish index . And in some species, the sexes are the same size. Like I said, there is no rule. Their ovipositors may enlarge. This is a tube from which the eggs flow at the fish's vent.
Males often have a more unusual shape.
They may have more adornments or bumps. Sometimes, a female can also have a few tubercles or a male may lack them. Males often chase females. In a few species, the female is the aggressor. This all depends on the species.