The length of pregnancy in a fish varies greatly depending on the species of fish. The length of pregnancy, or gestation period, can vary from as little as three weeks to ten months.
The striped seaperch is a species found in saltwater environments, and the length of pregnancy in this species can last as long as a human pregnancy with a gestation period of forty weeks, or ten months. However, in other species such as the molly fish, the gestation period can last as little as three weeks.
Common fish that are purchased as pets, such as goldfish, betta fish, and tetra fish, are all egg-laying fish. These egg-layers do not become pregnant. The type of fish that do become pregnant are categorized as live-bearing fish.
Guppies are a well-known variety of live-bearers. The female guppy, as with other live-bearing fish, can store sperm from the male guppy for months, sometimes as long as three months. After ovulation and fertilization occur, the length of time the female guppy remains pregnant varies from 21 to 40 days.
There are many live-bearing fish that can be purchased for the purpose of populating an aquarium. Molly fish and guppies are common live-bearing fish as well as swordtails, slaties and mosquito fish.
The length of pregnancy for live-bearing fish can also be minimally altered by environmental conditions such as water temperature and the availability of food. Check with an expert on your breed of fish for specific gestation periods.