How to Tell the Difference Between a Girl & Boy Goldfish

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Goldfish are one of the most popular fish to keep at home because they come in such a variety of sizes, shapes, and colors. More than 200 goldfish breeds offer a wide variety of colors and fin styles suited to just about any fishkeeper's fancy. No matter what breed, these easy to keep fish are readily gender-identifiable through physical characteristics and breeding behavior.

When you buy goldfish at the fish store, they are most likely 2 to 4 months old.
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When to identify goldfish gender

Like most fish, there is no discernible difference between male and female fry until they reach sexual maturity. When you buy goldfish at the fish store, they are most likely 2 to 4 months old, and they won't reach sexual maturity until they're 9 months to 1 year old. That means the cute fish you named Little Sally might actually be Big Sean when developed enough to breed.

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As they mature, both male and female goldfish will take on brighter colors than in their juvenile form. However, certain physical characteristics, such as the shape of their fins and the vent below their anal fin, will be clues to gender. The best time to identify your male and female goldfish is during breeding season. Goldfish mate when waters warm in the spring.

To emulate this warming cycle, gradually lower the temperature in the tank at a rate of about 3 degrees each day to prevent shocking your fish. Hold the temperature there for a few days and then gradually rewarm it at the same 3 degree per day rate to 70 to 72 degrees. Your fish will respond to the warmth with both physical and behavioral changes, indicating breeding readiness.

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Ready to spawn

Goldfish are peaceful, social tank inhabitants who spend much of their day sifting through gravel looking for bits of food. However, when they're ready to mate, their behavior changes. Male goldfish will follow a female around the tank, sometimes nipping at her belly to stimulate her to release her eggs for fertilization.

You can tell a male goldfish by the breeding stars on his gills and pectoral fins. These little, raised white bumps are a fairly reliable way to tell a breeding-age male, as he'll have a plethora of the small, uniform tubercules. Immature males and even some females might develop a few, but a breeding male will have cheeks covered in them.

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Female fish will develop a very round belly as they produce eggs inside that will be released for fertilization. A female goldfish can release around 10,000 eggs during a spawning session, making her swollen belly a telltale clue, especially when viewed from above. The eggs will float everywhere in the tank, sticking to pretty much everything. A breeding mop encourages goldfish to swim through the tentacles to spawn, making it easy to remove the eggs to another tank where fry can hatch and grow without being eaten by their parents.

Fin shape and goldfish breeds

Juvenile goldfish all have short, rounded fins, but fin shapes change in males when they reach maturity. The dorsal, anal, and pectoral fins become longer and more angular than those of females of the same age. Maturity at a year old is just an average. Slow-developing males might lag behind their fast-developing peers, appearing to be females for another couple of months.

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Fancy goldfish breeds can be more difficult to discern from fin shape, as the long, flowing fins appear identical in both genders. Look at the pectoral fins in these breeds. The male's fins are generally longer, thinner in width, and have a pointed tip. The female's front fins are shorter and wider, with a rounded tip.

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