How to Know When Gold Fish Are Ready to Mate

Updated February 21, 2017

Goldfish, or Carassius auratus, are colourful, hardy and lively fish. They are available in many varieties and colours, ranging from the sleek orange common goldfish to the exotic twin-tailed lionheads, elegant fantails, and inky black Moors. Goldfish, which can be kept in indoor aquariums and outdoor ponds, enjoy widespread popularity as pet fish. According to the Pet Education website, goldfish are the most popular domesticated fish in the United States. Breeding goldfish is not difficult. By providing proper conditions for your goldfish to mate, you can increase the odds of success.

Begin feeding your goldfish an enhanced diet to get them in optimal breeding condition. You should start this about two months before you would like the fish to mate. Augment good-quality commercial goldfish flakes with frozen or live food, such as brine shrimp and bloodworms. According to the Aquatic Community website, it is easier to distinguish the fishes' gender as they become ready to mate.

Look closely at the abdomens of your goldfish to determine their gender and their readiness to mate, and to make sure you are starting with a breeding pair. To increase your chances of accuracy, you should wait until the fish are over 3 inches long. Males will look thinner and sleeker in the abdomen, while the abdomens of females become fuller and heavier.

Observe the gills and front pectoral--or underside--fins for a scattering of pinhead-sized white pimples. These small dots, which closely resemble the lesions of "ich," or ichthyophthiriasis, are known as breeding tubercles; some aquarists refer to them as breeding stars. They are the sign of a healthy male fish that is ready to mate.

Examine the pectoral fins for another clue to gender. Males tend to have thickened pectoral fins.

Trigger breeding behaviour in your goldfish by slowly raising the tank temperature by -16.7 degrees C each day until it reaches 21.1 degrees Celsius. Goldfish, which are cold water fish, are normally kept at temperatures under 20 degrees C; 21.1 degrees C, however, is optimal for mating.

Watch your goldfish closely--especially in the early morning--for signs of the "spawning chase," the final, definitive sign that the fish are ready, willing and able to mate. The male will dash repeatedly towards the female's abdomen, sometimes nudging at the vent area. The female will lay the eggs in batches, and the male will spray each with milt, or seminal fluid. Eggs should be removed when the female is finished laying, placed in 6 inches of water in a separate 20-gallon tank, and kept at 22.2 degrees Celsius until they hatch.


If you have a goldfish that has been positively identified as a female, put it with other unsexed goldfish to help you identify their gender. You can tell by the other fishes' behaviour--nudging, chasing--which ones are male. Provide your goldfish with spacious living conditions if you want them to breed. According to the Aquatic Community website, 15 gallons per goldfish is a good rule of thumb. Keeping a higher ratio of males to females, such as 3 males for every 2 females, can increase the odds of breeding success.

Things You'll Need

  • High-quality commercial goldfish flakes
  • Fresh or frozen brine shrimp
  • Separate 20-gallon tank for eggs and fry
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