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Sexing Goldfish: 6 Ways to Tell the Gender of Your Goldfish

Oliver Jones

You don’t have to have intentions of breeding goldfish to be curious about the sex of your goldfish. Knowing the sex of your fish can help you understand behaviors your fish exhibit. A male chasing a female around the tank and nipping at her is likely breeding behavior, while a female chasing a male around the tank and nipping at him is likely bullying. To help you better understand and care for your fish, here are the things you should know about sexing goldfish. Maybe today will be the day you discover that Patrick the goldfish is actually Patricia.

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Is There a Trick to Successfully Sexing Goldfish?

There are two main things you should know when attempting to determine your goldfish’s sex. The first is that goldfish are not sexually dimorphic until they are adults. This means that when your goldfish is a juvenile, you will not be able to accurately determine the sex. Sexual dimorphism refers to physical differences between sexes, and even though goldfish become sexually dimorphic with age, the differences are minute. If you didn’t raise your goldfish from an egg, then it may be difficult to determine its age. Goldfish reach sexually maturity between 9-12 months of age. Most goldfish in the feeder tanks at the pet store are 2-3 months old, but you won’t know for sure until your goldfish actually reaches sexual maturity.

The second big thing to know about sexing goldfish is that the easiest time to try to solve this puzzle is to wait until breeding season. In the wild and in ponds, goldfish will become ready for breeding after the water begins to warm and they come out of torpor. In tanks, your goldfish may attempt to breed year-round if you don’t alter water temperatures to reflect the seasons.

veiltail goldfish in a tank
Image Credit: Hans Braxmeier, Pixabay

How to Sex Goldfish:

1. Watch for Behavior

Breeding behavior in goldfish is very distinct and makes it easy to determine who’s male and who’s female. Male goldfish will chase a female goldfish and nip at her back end in an attempt to stimulate her to release eggs. You may see multiple males chasing one female. This behavior can result in injury to the female as she attempts to escape the males, but it usually does not involve the males themselves damaging her fins. If you spot a fish that is chasing another and tearing at its fins, you’re likely dealing with a bully.

2. Check the Vent

Goldfish anatomy differs from mammal anatomy in many ways, and one of the big ways it differs is that goldfish do not have the same external excretory and sexual organs that mammals do. Goldfish have a vent, which is a body opening that releases waste and genetic material, be it eggs or sperm. Female goldfish tend to have a slightly out-turned vent, while males tend to have a flat or inward-turned vent. This means that if you view your goldfish in profile, a female will have a small bump where the vent is located, and a male will not.

Goldfish in a tank
Image credit: Pixabay

3. Breeding Stars

When male goldfish are ready to spawn, they will develop breeding stars. These small white flecks are often confused with ich because of their salt crystal appearance. However, breeding stars are concentrated on the gill plates and pectoral fins, while ich indiscriminately covers the body. Males use these breeding stars when they are chasing and nudging at the female’s vent to encourage her to release eggs.

4. Body Shape

When females are ready to spawn, they will begin to develop a more rounded abdomen as they produce eggs. Male goldfish will not have changes in their body shape when they’re ready to breed, so a goldfish that is slightly rounder and larger than usual is likely a female that is ready to spawn.

Ranchu Fancy Goldfish in tank
Image credit: Sardo Michael, Shutterstock

5. Fin Shape

Male goldfish and female goldfish have visible differences in their pectoral, or front, fins. These differences can be difficult to spot if you have particularly active goldfish. Males have longer, narrower fins than females, while female goldfish tend to have thicker, shorter fins. These differences are easier to spot in single-tailed goldfish than they are in fancies.

6. Spawning

If you witness your goldfish spawning, you’ll easily be able to determine the sexes. Female goldfish will release large quantities of orange eggs and the males will follow along behind her to fertilize the eggs. Females may begin releasing eggs while still attempting to escape from the males, so you may spot orange eggs in various locations in your tank.

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Conclusion

Determining the sex of your goldfish is not an easy task! Goldfish are busy fish and always seem to have somewhere to be. Getting them to hold still long enough for you to get a good look at things like vent and fin shape can be exceptionally difficult. Methods like watching for behavioral changes and spawning are far more reliable, but if you are not interested in goldfish fry potentially happening, then you may want to try to determine the sex before spawning begins to take place so you can separate your males and females. Expecting breeders to determine the sex of your goldfish at the time of purchase is unreliable and expecting the people at big box pet stores to be able to tell the difference is even more unreliable since they are less specialized than goldfish breeders. Understanding some of the sexual dimorphisms between male and female goldfish can make it much easier for you to determine the sex of your goldfish.


Featured Image Credit: Hans Braxmeier, Pixabay

Oliver Jones

Oliver (Ollie) Jones - A zoologist and freelance writer living in South Australia with his partner Alex, their dog Pepper, and their cat Steve (who declined to be pictured). Ollie, originally from the USA, holds his master's degree in wildlife biology and moved to Australia to pursue his career and passion but has found a new love for working online and writing about animals of all types.