Goldfish don't get pregnant; they're oviparous: The females swell with eggs, which they release in the water and connect with sperm released by the males. Goldfish breed when they're about 4 inches long. Both female and male goldfish show physical changes and changes in behavior before releasing their eggs or sperm, which is called milt. In the right conditions, goldfish breed easily, but often they eat the eggs before their owners notice they've been released.
How to Tell If a Goldfish Is Pregnant
A female goldfish looks pregnant when she's ready to release her eggs. In nature, mature female goldfish start to develop eggs in late winter and early spring. As the eggs swell, the females' bellies become fatter. Male goldfish also experience physical changes as breeding time approaches: They develop white bumps on their gill covers and pectoral fins, which are the fins on their sides. At the same time, the males begin to chase the females around the tank and bump the females' sides and bellies. This is to encourage the females to release the eggs so the males can fertilize them.
Male and female goldfish reproduce when water temperatures rise. When the temperature in the pond or tank reaches 68 degrees Fahrenheit, female goldfish release hundreds and sometimes thousands of eggs near aquatic vegetation. Male goldfish quickly release their milt in the same area to fertilize the eggs. Called spawning, this process usually takes place in the early hours of the morning and is over in three or four hours after spawning. Goldfish eggs stick to the vegetation or wherever else they land. Fertilized eggs are transparent and golden brown; unfertilized eggs are white.
After release and fertilization, goldfish eggs hatch in two to seven days. In water at 84 degrees Fahrenheit, fertilized goldfish eggs hatch in 46 to 54 hours; in water at 70 to 75 degrees Fahrenheit, they hatch in five to seven days. Goldfish fry carry yolk sacs that supply food for two or three days. When their yolk sacs are empty, they're ready to eat live food such as brine shrimp and daphnia. Goldfish don't look after their young fry at all -- in most cases the adults eat the fry whenever possible.
If you have a female goldfish full of eggs and a male goldfish, you can try to breed at home. Prepare a new tank for the goldfish fry and place a spawning mat or a new, clean mop head that hasn't been treated with chemicals in the aquarium tank with your breeding fish. Check your fish early every morning to see if they've spawned, and transfer the goldfish eggs on their spawning material to the new tank as soon as they appear. After 12 hours, check for white, unfertilized eggs and remove them. Feed the goldfish fry three times a day when their yolk sacs shrink, but take care not to overfeed them and pollute the water. Goldfish fry eat live food, powdered flakes and specialized liquid food.