In several other posts we have looked at fish breeding. We learned about fish that scatter their eggs and fish that hide or anchor their eggs. Fish that harbor their young within their bodies are called livebearers. This group of fish sometimes harbors the eggs in the mouth of the parent and sometimes in the abdomen.
There are about 4 species and a couple of sub-species commonly that are used in a fish tank aquarium. Included in this group are Mouth-breeding Betta, Moffat’s, Gunther’s Cichlid, and Black-chinned Mouthbreeders. In this method of spawning, the female drops her eggs and they are fertilized by the male. Then the eggs are picked back up and kept by one of the parent fish in the mouth until they hatch. The mouth of the parent is not only an incubation chamber, but after the young hatch, a place of safety to dash into in case of danger. Watching the young dash for safety can remind one of people rushing into the New York subway during rush hour.
Many more species of common fish tank fish retain the eggs within their abdomen, and include more than 20 species and several subspecies. Some of the more common to a home aquarium are the Guppy, Molly, Swordtail and Platy. Some of these species will inter-breed in captivity. This knowledge has given amateur and professional breeders a field day in producing many variations in colors and bodies that would not occur in nature.
Animals that retain fertilized eggs within their abdomens are called ovoviviparous. Eggs are produced and then fertilized internally by the male as he swims along beside the female, and the fertilized eggs are carried within the abdomen until they hatch. (This is not the same situation as found in mammals. Mammals not only retain the eggs, but set up a nourishment system from the mother to the young through the placenta. Ovoviviparous species simply retain the eggs and do not provide a maternal nutrition source.). Additionally, in some species the female fish can retain sperm cells for many months and as many as 8 broods have been produced from a single insemination.
As a general rule a well-fed mother will not eat her young. This does not say that other fish in a community tank will not feed on the newborns.
Fries can be produced and survive in a community tank. However, if the expansion of your fish population is your goal, a properly setup breeding tank will give you better results.