Some fish can begin reproducing immediately after birth, while others may take years to mature. According to Sea World, the smaller the size of an adult fish usually indicates a fish that will reproduce early. Age and size are said to be the main factors in reproduction and reaching maturation.

Livebearing

In this mating method, the female is fertilized by one of the male’s fins and will expel the live fry about a month later. Put down the ketchup — it’s not that kind of fry!Fry is the term used to describe a recently hatched fish and may also be called larval fish.
The young are fully formed and are able to swim as soon as they are released. One of their first goals is to search for food. An example of livebearing fish are guppies. Interestingly enough, once a female has hatched fry this way, she is able to continue to reproduce without a male present.

Egglaying

Other types of fish reproduce by laying eggs. The eggs grow until and hatch into fry in about 7 to 10 days but can vary. Some fish known as egg scatters will do just as the name implies: They scatter their eggs in various areas while the male follows behind and fertilizes the eggs. The two fish will race around the tank to complete this task without stopping to feed.
Substrate spawners will leave eggs attached to surfaces with saliva. This happens with the female laying the eggs and the male following behind to fertilize them. Catfish use this form of mating. Bubblenest builders are a little different. The male fish blows bubbles and the female lays the egg in this “nest” of bubbles located near the water’s surface. The location allows for maximum oxygen and attracts a food source. “Mouthbrooders” describes an incubation period done by either the male or the female. After the female lays the eggs and the male fertilizes them, one of the parents will scoop up the eggs in their mouth and keep them there until the fry are hatched.

Party of One

Some species need only one to reproduce — themselves! Unisex fish can reproduce without the other sex present. Unisex fish are always female and reproduce only females. They do mate with males of similar species, but the sperm never contributes to the heredity of the offspring. You may be wondering why they bother at all, but I’ll leave that to your imagination