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How to Tell if Your Goldfish is a Boy or a Girl?

Perhaps you’ve recently purchased a goldfish and are now wondering if it’s a boy or a girl? Or maybe you want to breed goldfish and need to know how to distinguish the males from the females when purchasing them from a pet store? Well, there are ways to physically determine the gender of your goldfish just by looking at it. These include the following:

Female goldfish usually have fatter, rounder bellies which is especially evident during pregnancy. Males, other hand, have jagged pectoral fins that are longer and thicker than those of females the same age. Males also have raised bumps on their gill covers and tend to be smaller and thinner than females. As well, females tend to be more brightly colored and usually swim faster than males.

Now that you know the main physical differences between male and female goldfish, let’s explore this topic further and in more detail below. I’ll explain if goldfish can change gender and how to tell if a goldfish is pregnant. I’ll also discuss which is bigger – the male or the female – and whether it’s best to keep goldfish in male/female pairs or in groups.

So, if you’re ready to learn more on male versus female goldfish, then let’s get started!

Can a Goldfish Change Gender?

While it’s entirely possible for some fish to spontaneously change gender, goldfish cannot. Apart from the obviously protruding belly or a pregnant female, it’s often difficult to tell a male from a female in an aquarium with multiple goldfish swimming about.

One sure-fire way to tell the males from the females is to check for breeding tubercles. These are prominent in male goldfish at breeding time and look like little white bumps along the gill cover. The males will also typically ‘chase’ the females around the aquarium as part of their mating ritual.

How Can You Tell if a Goldfish is Pregnant?

Pregnancy for fish begins in the spawning stage. This is when a female is preparing to lay her eggs. A pregnant goldfish will often become more sluggish and spend more time hiding near plants or rocks when she’s in the ‘nesting’ phase. She’ll also sometimes refuse food just before she drops her eggs.

When pregnant, you’ll notice the belly of the female getting larger and rounder. As well, if you place hand in the tank and pick up the fish, you’ll notice a bunch of eggs come streaming out of the female as she wriggles about. These eggs will hatch between 2 to 7 days after laying.

Are Female Goldfish Bigger than Males?

In most cases, female goldfish are slightly bigger than males of the same age. They’ll also look heavier in the abdominal region. Apart from the size difference, females tend to have brighter colour patterns and will often swim faster to avoid unwanted attention from would-be suitors.


Determining the gender of young goldfish is next to impossible, since both the males and the females look almost identical in size and shape – plus they’re so small, it’s difficult to distinguish one physical feature form the next.

If you want to breed goldfish, you should consider purchasing at least 6 healthy youngsters of the same species at the same time. That way, there’s a 98% chance (statistically speaking) that you get at least 1 fish that’s of a different gender than the rest.

What are the Differences between a Male and a Female Goldfish?

The 5 main differences between male and female goldfish include the following:


  • Females of the same age are slightly bigger than males with larger, more circular-shaped ventricles that become swollen and puffed out during the mating season.
  • Males have longer, thicker pectoral fins than females.
  • Males also tend to have longer, more pointed tail fins.


  • Females tend to be rounder and thicker than males, especially in the abdominal region.
  • Males have a more angular body structure.
  • Males look elongated and lithe compared to females.
  • Males are more streamlined than females.
  • Females will also have a bulge on one side of their bodies where the eggs are developing, making them appear more asymmetrical or lop-sided.


  • Females are usually more vibrantly coloured than males.
  • Males have longer, thinner pectoral fins that appear more ragged.
  • Males have visible white bumps along their gill covers.
  • Males have a small, oval-shaped anal opening whereas for females, this feature tends to be larger and protrudes slightly.
  • Males have a raised line running along their underbelly whereas females don’t (or if they do, it’s very faint).


  • Females are often calmer and less aggressive than their male counterparts.
  • Females will often hide among plants and rocks prior to spawning.
  • Females will sometimes refuse food prior to spawning.
  • Males usually flit or dart around the tank as part of the breeding process.
  • Males will often ‘chase’ females around the tank and even pump into them occasionally to encourage them to mate.


  • Females will often swim faster than males to ‘outrun’ a pesky breeding partner.
  • Males will swim faster than pregnant females, who become sluggish prior to spawning.
  • Males will chase other males around the tank in the absence of females, whereas females usually won’t chase other females.

When in doubt, you can always seek the advice of a veterinarian to correctly determine the sex of your goldfish. If you notice the white tubercles on your goldfish, this is natural for males and don’t take it to be a sign of illness. Inexperienced aquarium hobbyists often rush to judgment and start administering drugs for what they assume is a bacterial or parasitic infection.

Is it Best to Keep Goldfish in Pairs or Groups?

While you often see goldfish swimming alone in a bowl or small aquarium in pet stops or fish stores, they’ll be happier, healthier and live longer if kept in pairs (or groups) in a larger community tank with other fish of similar size and temperament.

Ideally, if you want to breed goldfish, then you should keep them in pairs with 1 male and 1 female per tank. The tank should contain at least 20 gallons of water (per pair) with a filter that’s cycled regularly. To increase the chances of successful spawning, you may consider keeping more than 1 male at a time.

However, if you decide to keep multiple male goldfish in the same aquarium, then be sure you have more than just 1 female since this can increase tension and aggression in males who may ‘fight’ amongst each other for the right to breed with the lone female. The best ratio is 1 male for every 2 females.



To conclude, there are obvious physical difference between male and female goldfish. Females usually have bigger bellies, which is a telltale sign of pregnancy. They also tend to be more brightly coloured and will often swim faster than their male counterparts. Males have longer, thicker pectoral fins with raised while bumps along their gill covers. They are also generally smaller and thinner than females of like age.

I trust this article has been of help to you. Thanks for reading and good luck with your goldfish aquarium!

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