If you want a pair of cichlids, buy 8-10 young fish and grow them up together and let them pair off naturally. Some shops will sell some cichlids as pairs (usually Apistogramma sp) but most species are just sold as is (you get what you get unless you specifically ask, and the shop knows how to sex them).
If you buy 10 tetras, barbs, danios, rasboras or Corydoras, you should get males and females in the group. All these fish should be kept in groups of 10 or more. For tetras, barbs, danios and rasboras, the females are usually fatter and the males are usually slimmer. If you look down on adult Corydoras, the females will be bigger and wider, males are shorter and slimmer.
Rainbowfish can sometimes be bought as pairs but again, just buy 8-10 and you will get males and females. Or ask the shop to get you some males and some females, or pick the fish you want. Males have more colour (or different colour) and longer more pointed dorsal and anal fins. Females usually have less colour, don't get as high in the body and have shorter more rounded dorsal and anal fins.
Goldfish can't be sexed until they are at least 1 years old. Females are usually fatter than males. Males get small white dots (breeding tubercles) on the gill covers and pectoral fins when in breeding condition.
A lot of male suckermouth catfish will develop small hairs/ bristles on the face and pectoral fins when they mature, but not until they mature. If you want to breed these types of fish, buy a group of 8-10 and grow them up.
Most common livebearers (swordtails, platies, guppies & mollies) can be sexed quite easily by looking at the anal fin. Females have a fan or triangle shaped anal fin. Males have a long thin anal fin.
Killifish are often sold in pairs but it depends on the shop.
Gouramis can usually be sexed when mature but not when young. Adult males usually have longer more pointed dorsal fins and bigger anal fins. Males of some species have different colours to the females or brighter colours.
