In outdoor ponds, goldfish will readily breed in captivity. Even in the home aquarium, a particularly well-cared for goldfish may spawn. You need to recognize the signs that the goldfish is preparing to spawn, since you may have to make arrangements to care for the baby fish.
All About Timing
Timing can help you tell if your goldfish are about to spawn. First, goldfish kept outdoors generally spawn in the springtime, while indoor goldfish tend to spawn year round. Also, goldfish will not spawn until they are at least a year old.
Watch the Males
Male and female goldfish kept under similar conditions tend to get ready to breed around the same time, so if you see these signs in your males, it's a good sign the females are getting ready to breed as well. Male goldfish develop small white bumps called spawning tubercles on their heads, gill covers and pectoral fins when getting ready to spawn.
Female goldfish also change a little when preparing to spawn. Female goldfish always have a more rounded body than their male counterparts. But when they get ready to spawn, female goldfish swell up with eggs, causing a visible swelling.
Right before spawning, male goldfish begin to chase female goldfish. Hobbyists call this the spawning chase. In fact, this behavior is the most reliable way to tell male goldfish from females, since spawning tubercles may be not be obvious in certain individual and females' more rounded shape is somewhat subtle.