A mix of male and female fish

This may sounds obvious, but some fish are not easy to sex. In species that are difficult to sex, is best to start out with at least six young fish so that you are certain of getting both males and females. Starting with many fish also gives monogamous fish a chance to pick compatible mates. Sometimes if a single male and female are introduced, they will not breed. Other fish, like livebearers, killifish, and polygamous cichlids need more females that males so that females are not harassed by amorous males.