There are two types of breeding fish: live bearers and egg layers. Goldfish and marine fish are all egg layers only, while tropical fish have both live bearers and egg layers. Live bearers are by far the easiest to breed and are ideal for the beginner.
The most popular live bearers are the large, active tropical fish, which include Swordtails, Platys and Mollies. These all require plenty of space. Live bearers are highly inbred and although inbreeding is necessary to fix the beautiful strains, it also has undesirable effects. Live bearers, if not properly cared for, may suffer from diseases such as ich, shimmy, and various skin diseases. Should the tank temperature fall, the live bearers are usually first affected. They are active fish, both sexually and in growth potential. Live bearers usually come from harder, more alkaline waters than egg layers. Poorly managed tanks tend to become more acid and hence cause live bearers problems. Live bearers can be kept at lower temperatures (22°C). Those raised at slightly higher temperatures mature more quickly, but their lifespan tends to be shorter.
The water pH for live bearers should be 7.0-7.2. Hardness is tolerated by live bearers but it should be kept at approximately 200 p.p.m. General aquarium maintenance as discussed elsewhere in this chapter is most important.
The sex of live bearers is easily distinguished by the presence in the male of the gonopodium. All young live bearers have a fan-like anal fin, but as they mature the male’s begins to change shape to the typical narrow ‘sticklike’ fin (gonopodium). This is usually carried close to the body, while the female anal fin is spread out. By means of a series of specialized muscles, the gonopodium can be moved and inserted into, the female for fertilization Live bearers’ eggs are not discharged from the ovary until long after fertilization has taken place, and just before the fry are fully developed and ready to be born.
Guppies, Swords and Platys have a regular brood production and will drop young at intervals of twenty-five to thirty-five days, depending on temperature and lighting conditions.
Mollies have irregular brood production and are greatly affected by seasonal or artificial changes in temperature or light. However, since most aquariums provide constant conditions they usually deliver at constant intervals.
The number of young delivered at one time is variable and depends on the age of the mother as well as on her size. The young are about 5 millimeters in length and may number from six to 200 or more. Mollies usually number between twenty and forty, while Swords and Platys number sixty to eighty. Guppies average twenty-five to thirty.
Live bearers have the ability to store the sperm in the female oviduct for tip to six months, which means that five or six broods at monthly intervals can be produced without further contact with the male.
Fish selected for breeding should be placed in their own quarters and fed well. Since there is no need for the male after fertilization, he should be removed as delivery time approaches, because he will eat the young. In all but dark colored live bearers, a dark area can appear on the female’s body near the vent as she fills with eggs. This is known as the `gravid spot’. It should not be used as a reliable indication of the fish’s sex.
Protection of Newborn Fry
Since newborn fry are often eaten by adult fish in the aquarium, it is important to provide some form of protection for them. It can be provided mechanically or naturally.
The mechanical method involves the use of devices known as breeding traps which are usually small containers, partially submerged in an aquarium, and which have small openings or slots at the bottom. The female is placed in the containers and the young, which do not swim immediately