Breeding and Reproduction of Fish - All Other Pets - MSD Veterinary Manual (2024)

Determining the sex of a fish can be difficult or easy, depending on whether physical differences are visible. Males of some species may be larger and showier than females. Information on how to sex a particular species may be obtained from your veterinarian, books, hobby magazines, the Internet, and other sources.

Fish reproduce by bearing live young or by laying eggs. Livebearers give birth to fully formed and functional young called fry. The eggs are fertilized and hatch within the female. Most livebearers have fewer and larger fry than egg layers because the fry need to be more developed and large enough to fend for themselves after birth. Most species of livebearers kept in home aquariums are generally easy to breed. Identifying sexes is usually easy as well. Males are generally larger and have larger, longer, more ornate, and more colorful fins than females. For instance, only male swordtails have the “sword” on their tails, and male guppies have larger, more flowing tails that are brightly colored.

Fry should be separated from adults because the adults (including the parents) tend to eat them. Small live or frozen food and crushed flakes are good for feeding fry. Species of freshwater livebearers include mollies, platys, swordtails, and guppies from the Americas and the 20 or so halfbeak species from Asia.

Egg layers spawn by several means, including egg scattering, egg depositing, egg burying, nest building, and mouthbrooding. In all cases, eggs are laid and fertilized outside the body. Nest-builders and mouthbrooders are generally good parents, protecting the eggs and fry from aggressors. Many cichlid species, such as freshwater angelfish, are nest-builders.

Egg scatterers, egg depositors, and egg buriers may or may not defend the eggs and fry. Usually the fry need to be separated from the adults to prevent the larger fish from eating them. Egg-scatterer females lay sticky eggs in various places within a certain area (often in areas that provide some sort of cover), while others set nonsticky eggs adrift in open water. Egg depositors pick one general spot to lay sticky eggs, usually on the bottom substrate and sometimes on the aquarium glass. In salt water, clownfish are depositors, guard their eggs and fry, and are the most likely species to be bred by hobbyists. Egg buriers either dive into soft substrate or the male pushes the female into the soft substrate to lay. The male then dives in to fertilize the eggs. In a tank breeding environment, peat moss is often a good choice for the substrate.

Usually fry should be separated from the adult fish and placed in a nursery environment. Mouthbrooders will eventually expel the fry even when the fry are still quite vulnerable. Removing fry from outdoor ponds can be difficult. Ideally, a separate, smaller aquarium should be set up to receive them. Conditions should generally be kept much as they are in the main aquarium or pond. There should be some kind of cover for the fry so they are safe, secure, and free from stress. The aquarium should be filtered, but the pump should not be so powerful that it sucks in the fry. Several commercial baby fish foods are available. Alternatively, finely crushed flake and tiny live or frozen foods can be fed.

Another option is to purchase a nursery. Nurseries are made of a box frame with a fine mesh netting for the walls and floor, or plastic grids for the same purpose. They are usually built to hang from the top lip of the aquarium into the water. The mesh or grid prevents the fry from escaping while keeping them safe from the larger fish. The open top allows access for feeding and other purposes.