[header] How to breed fish [title] Pick out a healthy adult fish of each sex. [step] In order to breed fish, your first step is to determine their biological sex. Unlike some other animals, this will be trickier than looking for differences in their genitalia. 
The biological differences between species can vary greatly, but there are a few common traits to look for. Female cichlids, for example, tend to have rounded bodies and less pronounced vents, while males usually display more vibrant coloration and sometimes have a small hump on their heads.