The Goldfish Tank explains that a female goldfish lays hundreds of eggs after mating with a male goldfish. This is also called spawning. However, goldfish are not easy to breed. A goldfish will rarely lay eggs in a home aquarium, according to The Goldfish Tank. In order to have babies, a goldfish has to be in a special environment that is difficult to replicate except in outdoor ponds.
According to Goldfish2Care4.com, goldfish are only able to spawn once each month from April to August. If the water conditions are right, the female releases pheromones that inform the male goldfish of her ability to breed. The male goldfish then pursues the females in a way that appears aggressive. In some cases, the males become too persistent and need to be separated from the females to prevent injuries.
Goldfish2Care4.com states that the male goldfish will nudge the female's stomach to encourage her to release her eggs into the water. After the eggs have been released, the male fertilizes them. The eggs stick to stones and plants until they hatch. However, Goldfish2Care4.com also explains that only a small portion of the eggs will be fertilized, and not every fertilized egg will grow into a healthy adult.