While the exact length of the gestation period for a tropical fish varies from species to species, the average pregnancy lasts about one month, give or take a week.
Livebearers such as guppies, mollies and swordtails are freshwater tropical fish that do not lay eggs. A female livebearer gives birth to miniature fish that are free swimming from the start. A male livebearer fertilizes the eggs inside the female just as mammals do. Near the end of the gestation period, the female will swell significantly in size. Due to the translucency of the scales on some types of tropical fish, it is possible to see the smaller fish inside the pregnant female.