How to Know if a Goldfish Is Male or Female: Physical & Behavioral Signs

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Plus, how to encourage breeding & tell when goldfish are mature
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Curious about the sex of your goldfish? Whether you want to breed your fish or just need to double check that you gave them a suitable name, determining the sex of a goldfish is possible through close observation of their physical traits and behavior (especially during spawning season in the spring). In this article, we’ll show you all the important differences between male and female goldfish and why it can sometimes be a challenge to sex them accurately (even for the pros!).

How can you tell the sex of goldfish?

Female goldfish are rounder and thicker, have a richer color, and have shorter fins than males, which are more streamlined, paler, and have longer, pointier fins. Females have a protruding anal vent, while male vents are concave. Males develop white spots (tubercles) and show chasing behavior during spawning season.

Section 1 of 4:

Identifying Female Goldfish

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  1. 1
    Look for a rounder, thicker body shape. Female goldfish tend to have a rounder, thicker body shape than males of the same age and species. They also tend to be deeper-bodied (longer from back to belly), which is useful for identifying sex from a side view. [1]
    • As breeding season approaches, the females will begin to develop roe (eggs), which may cause a bulge on one side, making the female look asymmetrical or lop-sided from above.
    • Note that the shape of the fish is what’s important, not the size. There is no clear difference in the size of male and female goldfish. [2]
  2. 2
    Look for a protruding vent. The vent (anal opening) of a female goldfish is rounder than the male's and tends to protrude from the body slightly as breeding season approaches. It is convex, or an “outie.” [3]
    • When viewed from the side, the vent may appear as a raised surface on the female's abdomen.
    • Outside of breeding season, the female’s vent may be less noticeable and harder to distinguish from a male’s.
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  3. 3
    Check for shorter, rounder fins than males. Goldfish have several sets of fins, but there are 3 fins that can help you sex them—2 pectoral fins (one on either side) and 1 anal fin (on the underside, near the vent). All 3 fins are shorter and rounder on females compared to males. The anal fin is also typically thicker than a male's. [4]
  4. 4
    Notice if the fish has a bright, vibrant color. In general, female goldfish have a richer, brighter color than males, who tend to be slightly paler and duller than females. Observe all of your goldfish at once and check for any fish whose color jumps out at you. Chances are it may be a female. [5]
    • Goldfish come in many colors, with the iconic orange-gold hue being the most common. Even if your goldfish aren’t “gold,” the females are typically more boldly or brightly colored.
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Section 2 of 4:

Identifying Male Goldfish

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  1. 1
    Look for tubercles (“breeding stars”). One of the main tell-tale signs that your goldfish is male, is the development of small white spots called tubercles on their gill-shields. [6] Usually, the breeding tubercles will only last during the fish's breeding period. However, in older males who have lived through several breeding seasons, the tubercles may be present all year round.
    • The tubercles may also appear on the pectoral fins, on the face, and on the scales.
    • The presence of tubercles is a good indication that your goldfish is male, but their absence does not necessarily mean that your fish is female since not all males will develop tubercles.
  2. 2
    Look for a thinner, more streamlined body shape. Males tend to have longer, thinner, more streamlined body shapes than females of the same age and species. [7]
  3. 3
    Check for a concave vent. A male goldfish's vent is usually narrow and elongated, making it somewhat ovular in shape. It will also be concave (an “innie”), rather than protruding like a female’s [8] .
  4. 4
    Check for longer, more pointed or angular fin shapes. Males will have longer, more pointed pectoral fins (the fins on the goldfish’s sides) compared to females. The anal fin (on their underside) will also be noticeably longer and more pointed compared to a female’s anal fin, which will be smaller, curved, and thicker. [9]
  5. 5
    Look for a midline ridge. If possible, look at the underbelly of the fish to see whether it has a midline ridge (a raised line running from the back of the pelvic fins to the vent). In females, this line will either be very faint or non-existent. [10]
  6. 6
    Watch for chasing behavior during spawning season (late spring). One of the most reliable ways to identify male goldfish is to observe their behavior during spawning season.
    • A male goldfish will chase a female around the tank or pond, staying close behind and slightly beneath her, sometimes nudging her rear. [11]
    • The male will also attempt to push the female against the side of the tank or into any plants, in order to force her to spawn. This spawning chase may last for several hours at a time.
    • However, in the absence of females, male goldfish will also chase each other around to assert dominance, so it's a good idea to use physical indications as well as behavioral ones to identify sex.
    • Outside of the spawning season, female goldfish are usually more active and energetic than males.
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Section 3 of 4:

Why is it challenging to sex goldfish?

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  1. 1
    Sex differences only appear once the goldfish are mature (9-12 months). The differences between male and female goldfish will only become obvious once the fish reach maturity, which usually takes about 9 to 12 months. [12]
    • However, the rate of maturation will depend on species and sex. Some breeds of male goldfish will reach maturity by the time they are 9 months old, while some females may take up to three years to reach maturity.
    • Identifying the sex of young goldfish is practically impossible. If you wish to increase your chances of having both male and female fish, your best bet is to purchase a large batch of fish—at least six healthy ones of the same variety. Statistically, there is a 98% chance that at least one fish will be a different sex from the others.
  2. 2
    Sex identification methods only apply to healthy, well-fed goldfish. Unhealthy goldfish may not follow regular spawning behavior or develop the usual physical characteristics that indicate their sex. So, it’s important to keep your goldfish healthy, feed them an appropriate amount of nutritious food, and make sure they have good quality water to live in before you attempt to identify the sex.
    • For example, an unhealthy male goldfish may not develop the tubercles usually seen during spawning season, while an unhealthy female may not have a convex vent.
    • Body shape can also be misleading. A thin goldfish may be mistakenly identified as male (as males are usually smaller), but you may be looking at an underfed female. On the other hand, you may read a swollen abdomen as a sign that the fish is female, but the swelling may be a sign of dropsy (an internal bacterial infection). [13]
  3. 3
    The only foolproof way to sex goldfish is to observe them spawning. Accurately identifying goldfish as male or female can be difficult, and even the experts get it wrong from time to time. Ultimately, a DNA test or watching them mate is the only surefire way to tell. [14] This is due to the fact that there are many exceptions to the rules:
    • Some male goldfish will not develop tubercles, while in rare cases a female goldfish will. [15] Some female goldfish will not have protruding vent, while male goldfish sometimes will.
    • In addition, some breeds of goldfish do not comply with “the rules.” For example, some breeds (such as ranchu goldfish or ryukin goldfish) naturally have very round, thick body shapes, which makes gender identification through body shape almost impossible.
    • As a result, it is best to identify the sex of your goldfish by observing multiple signs, rather than trusting a single sign alone.
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Section 4 of 4:

Frequently Asked Questions

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  1. 1
    Can goldfish change from male to female? No, goldfish are a non-sex changing fish under normal conditions. However, some research indicates that goldfish can begin behaving like the opposite gender when exposed to hormonal treatments (without losing the sexual function of their sexually mature gender). [16]
    • Some anecdotes claim that isolated groups of goldfish may have some members change gender in order to reproduce, but more observation and study is needed to see if these claims are accurate.
    • Other species of fish that can change gender include the kobudai (Asian sheepshead wrasse), clownfish, and many species of parrotfish and reef fish. [17]
    • Most sex-changing fish switch from female to male, although there are some species that can switch from male to female (and some that remain hermaphroditic throughout their lives).
  2. 2
    How can you tell when a goldfish is an adult? “The only way to really know that is when it becomes sexually dimorphic, meaning it has visual characteristics that allow you to determine whether it is male or female,” explains aquarium expert Brian Blake. “Once they develop those characteristics, then you can identify them as male or female and safely say it is an adult.”
    • Blake continues, “Another way you can tell is by their behavior. If you have a male and female together and they start courting and locking lips and trying to make a nest, then you can assume that they are also adults.”
    • The word “adult” may be hard to define for goldfish, though. According to Blake, “Goldfish can grow very large and can live 15 to 20 years, only reaching their maximum size in those final years. By that definition, adulthood would be pretty indeterminate, meaning it never really begins or ends. But I think the most classical definition would be once it becomes sexually mature.”
  3. 3
    How can you encourage goldfish to breed? “Goldfish breed in the spring,” explains aquarium specialist Craig Morton. To mimic the spawning temperatures of spring, he suggests “gradually lowering your aquarium temperature to 52°F (11°C). Then, when you are ready to breed, raise the temperature 3 degrees per day until it reaches 72°F (22°C).”
    • To encourage breeding even more, supplement your goldfish’s diet with meaty treats like mysis shrimp or bloodworms, or switch to a high protein goldfish food to help them put on some weight a few weeks before you want them to breed. [18]
    • Add some bushy plants (real or fake) or a spawning bush to your tank so the goldfish have a comfortable place to lay their eggs.
  4. 4
    How can you tell if a goldfish is pregnant ? Technically, a female goldfish can’t be “pregnant” because it doesn’t have a uterus and it releases its eggs to be fertilized externally. However, you can tell when your female goldfish is beginning the spawning cycle when: [19]
    • Its abdomen begins to look larger, indicating the presence of eggs.
    • It appears asymmetrical from above as the eggs clump to one side or another.
    • It behaves more lethargically and moves slower than normal.
    • It lingers near aquatic plants because they provide shelter for eggs.
    • You see eggs in the tank.
    • Note that if your goldfish live indoors and you haven’t manipulated the water temperature to mimic the warming water of spring, their bodies may not register that it’s time to breed.
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Expert Q&A

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  • Question
    What conditions do goldfish need to breed?
    Craig Morton
    Aquarium Specialist, Aquarium Doctor Inc.
    Craig Morton is the CEO of Aquarium Doctor Inc. based in Huntington Beach California and servicing Orange County, Los Angeles County, and the Inland Empire. With over 30 years of aquarium experience, Craig specializes in creating custom aquarium designs along with aquarium installation, service, and maintenance.
    Craig Morton
    Aquarium Specialist, Aquarium Doctor Inc.
    Expert Answer
    Goldfish breed in the spring and need the temperature to be adjusted. First, lower aquarium temperature to 52°F (11°C). Then, when you are ready to breed them, raise the temperature 3°F (1.7°C) per day until it reaches 72°F (22°C).
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  • Question
    How long does a goldfish live for, on average?
    Community Answer
    Sadly, the average household goldfish lives about six months because no one does the research on how to help them live longer. Goldfish can actually live up to 20 years and almost every species can grow over 6 inches (15cm). Goldfish can't live in goldfish bowls because it doesn't allow them sufficient room to grow. It also doesn't allow for decorations to be added, and goldfish are prone to depression because they are very curious fish and when there is nothing to enjoy, they'll pine. Always make sure your goldfish has enough room to swim. Compare it to yourself, would you like to live in a bowl the size of your bed? Also, there are different types of goldfish and the more streamlined bodied ones shouldn't be put with rounded bodied ones because they swim faster and will eat all the food.
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  • Question
    I have a goldfish, and it's chasing the other four fish. Is it a boy or a girl?
    Community Answer
    If it is nipping the tails of the others, then this is not a act of breeding, and it could be either. But if he swims under the others and noses their bellies, this is a sign of a male fish during breeding season.
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      Tips

      • Try going to a pet store and looking at larger goldfish. This could help you learn to tell the differences between them more easily.
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      Tips from our Readers

      The advice in this section is based on the lived experiences of wikiHow readers like you. If you have a helpful tip you’d like to share on wikiHow, please submit it in the field below.
      • When selecting new goldfish, observe their activity level and overall health. The most active, feisty fish often adjust best to a new home and live longer. Talk and interact with them often, too.
      • Don't assume the larger goldfish is male. Females are often larger, especially when mature and carrying roe. Use multiple indicators to determine sex instead of size alone.
      • Purchase young goldfish from the store, as they tend to be healthier and will live longer. Older fish may have undisclosed health issues that are difficult to resolve.
      • If one goldfish seems to be chasing, nudging or closely following others in the tank, this likely signals breeding behavior. The one pursuing is probably male.
      • Never keep goldfish in small bowls, which essentially suffocate them. Always provide an adequately sized tank with filtration to maintain water quality.
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      References

      1. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      2. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      3. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      4. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      5. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      6. ↑ https://www.fishkeepingworld.com/how-do-fish-mate/
      7. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      8. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      9. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      1. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      2. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      3. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      4. ↑ https://www.petmd.com/fish/conditions/urinary/dropsy-fish
      5. ↑ https://a-z-animals.com/blog/male-vs-female-goldfish/
      6. ↑ https://www.about-goldfish.com/The_Goldfish_Gazette-September-2019.html
      7. ↑ https://www.researchgate.net/publication/287277401_Sexual_plasticity_of_behavior_in_goldfish
      8. ↑ https://www.bbcearth.com/news/fish-are-the-sex-switching-masters-of-the-animal-kingdom
      9. ↑ https://cafishvet.com/goldfish/how-to-breed-goldfish-successfully/
      10. ↑ https://pangovet.com/pet-health-wellness/goldfish/how-to-tell-if-goldfish-is-pregnant/

      About This Article

      Article Summary X

      To identify a female goldfish, look for a rounder, thicker body shape. Females may also have a bulge on one side as breeding season approaches, and their vent, or anal opening, may protrude slightly. Male goldfish often have a thinner, more streamlined body shape than females, with a concave vent and a midline ridge. Also, males often develop small white spots called tubercles on their gills or face during breeding season. If you want to learn how to tell your goldfish's sex by their behavior, keep reading the article!

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