Livebearers include some of our most popular aquarium fishes; the Guppies, Platies, Mollies, and Swordtails. These fish are not critical of water conditions. However, they do not like peaty water. Most do well in slightly alkaline, moderately hard to hard water at a temperature between 72-80°F.
The males and the females are easily distinguished by the difference in their anal fins. In the male this fin is modified to form a rod-shaped copulatory organ called the gonopodium. This change occurs either early or late in life, at birth and for some time after that the anal fin of both sexes is similar. The gonopodium is very mobile and can be moved forward, backward, and sideways in most species. Once a female has been mated with a male, she can store the sperm and deliver a number of successive broods without further contact with a male. The mature female is easily recognized by a swelled abdomen and a large, usually black or grey spot in front of the anal fin.
This feature is referred to as the gravid spot. In the Black Molly, of course this gravid spot is not visible. In albino variants (ie. red-eyed Red Swordtails), this spot appears pinkish in colour. On closer examination of the gravid spot (preferably with a light situated behind the fish), spherical translucent structures (eggs or developing embryos) can be seen in this area. The gravid spot is not an absolute indication that a fish has been fertilized, for even virgin females at times show it.
The gestation period, the time interval between the event of fertilization and the delivery of the young, is highly variable. Although a broad generalization would be 4-6 weeks, at a temperature of 78-80°F. The number of young produced is also variable, it depends on the species and the size and age of the female. Livebearers with similarly shaped gonopodia can be hybridized. Thus Platies will cross easily with Swordtails, and Guppies have been crossed with Mollies.
The fact that one fertilization can suffice for the production of numerous successful broods must be kept in mind when selectively breeding livebearers. A female that is to be kept for mating must be isolated from all other males, if this is not done – even if a desired male is introduced to her later, one cannot be certain of the male parentage of the future progeny. Thus, in selective breeding the first step is to isolate breeding stock at an early age, before signs of gonopodium elongation. Then when the separated males and females are mature, the best male is mated to two or three of the best females by bringing them together in a well planted tank with surface vegetation. If fed well, nothing more needs to be done, the fish will readily mate. When the fry are born, catch the youngsters and rear them in a separate tank. The fry may be feed newly hatched brine shrimp and finely ground flake food.