The organisms we call fish include a huge variety of creatures. Fish belong to at least three classes -- for comparison, all mammals belong only to one. Within this vast diversity, fish reproduction varies wildly. Some species grow quickly to their full size, while others take decades. The reproduction of fish greatly influences how they interact with their environments, and it has conservation implications.
There are two ways to determine maturity in fish: adult size and sexual maturity. However, this breaks down for some fish. Many species can reproduce before they reach their full size. Additionally, some fish can hold off on reaching full sexual maturity until environmental conditions are favorable to reproduction. Many harem-spawning fish like clownfish follow this pattern. So the term "fully grown" can mean several different things, and might not be the best way to describe the life history of some species.
Small, minnow-like fish can reach maturity in less than two months. This includes livebearing fish like guppies, as well as egg-layers like danios and rasboras. In this case, these fish may reach both their adult size and sexual maturity at the same time, meaning they are both fully grown and sexually mature in less than two months. Some external factors influence how fast they grow and mature, including temperature and how much food they eat.
Generally, larger fish take longer both to reach their full size and maturity. For example, the freshwater sturgeon fish may take up to two decades to reach sexual maturity. The even larger whale shark is believed to take 30 years before it can reproduce. Both species will continue slowly growing for years after this point. With very large wild fish, it is not always known how long they live, and numbers about growth and maturity are ultimately estimates, albeit educated ones.
The differences in reproduction often influence the way fish interact in the wild. For example, small, fast-breeding species like guppies and their close relatives the mosquitofish often become invasive, since they can reproduce quickly. Their short generational spans allow them to quickly adapt and multiply when someone dumps them in a new environment. Similarly, larger, slow-breeding fish face a greater chance that they'll become endangered, since it takes longer to replace members of the species lost to hunting or habitat loss. This has happened with sturgeons, which are hunted for their eggs or caviar.