good chance of not actually having a mix of sexes.
Chances are 1 in (2[sup]n[/sup]/2), where n is the number of fish
If there are 2 fish, then 2[sup]n[/sup] = 2[sup]2[/sup] = 4, then 4/2=2, ie 1 in 2 chance (or 50:50), so a very good chance of not having a mix.
If there are 5 fish, then 2[sup]5[/sup]=32, 32/2=16, so a 1:16 chance of all the shoal being one sex or the other, so a very slim chance of not having a mix.