Goldfish reproduce by laying eggs that are then fertilised. The signs of a pregnant goldfish are visual, so it is important to have a baseline idea of what your goldfish looks like before it gets pregnant to determine whether or not it is ready to lay eggs. A mature goldfish will breed two to three times a year, so most goldfish owners interested in breeding will quickly learn to see the signs of a goldfish ready to breed.
Determining whether a goldfish is male or female mainly comes from the size and shape of the goldfish. Females are generally larger than males, and the midsection is heavier. To the touch, the goldfish will be squishier in the middle. This is due to the abdomen being full of eggs. The pectoral fins, which are on the sides of the chest, similar to arms on a person, are rounded and shorter on females than on males. The anal opening is also larger and protrudes slightly. When breeding begins, the male will also chase the female, which can help determine the sex.
Goldfish will not breed until they are at least 1 year old. As goldfish can live to be 10 years old, they have many years to potentially breed. The age can be difficult to determine if the fish is not bought when young. The size of the goldfish is dependent on the size of the tank, and small and large goldfish can breed, so without knowing the age of the goldfish beforehand, it is hard to see if the fish is ready to reproduce. Generally, the physical changes in a goldfish are better factors for determining the ability to reproduce.
As they lay eggs, female goldfish do not become pregnant, but they do fill with unfertilised eggs. When full of eggs, female goldfish will plump in their midsection. Male fish will develop breeding tubercles on their gill plates, as well as small white spots on their pectoral fins that show they are ready to breed. The breeding tubercles can be mistaken for Ich, which is a disease common to goldfish. The best way to determine whether the white spots are breeding tubercles or Ich is to check the rest of the fish's body. If the spots are only on the gill plates, then they are tubercles. If they appear throughout the body, then it is Ich.