Most tilapia farmers prefer to raise mono-sex cultures as male tilapias grow faster and bigger than females, making the farming more profitable. This means that it is important to be able to sex Tilapia. Farmers want to sex tilapia at as young an age as possible to be able to separate and destroy the females. (Today, it is also possible to obtain all male or almost all male batches of tilapia by purchasing certain hybrid strains or by exposing the young fish to hormones.)
Unfortunately, tilapia can not be sexed as easily many other cichlids and there are no obvious external differences between male and female tilapia. It can therefore be hard to sex them and only specially trained individuals are able to do so and even they tend to get at least 2-5 percent wrong.
Tilapia can usually be sexed once they reach 25 grams (1 ounce). The way to do this is to look at the genital papilla located behind the anus of the fish. If it has one opening it is a male tilapia. This one hole is used to pass both urine and milt. If there are two holes it is a female. The females use one hole to pass urine and the other to lay eggs.
You can highlight the area and make sexing easier by putting one drop of dye, such as food colouring or methylene blue, on the genital region. The colour makes it easier to spot the openings.