Male vs. Female Goldfish

Sexing your goldfish can be important if you own more than one fish, which is probably best to prevent them from getting lonely. Keeping one female with multiple males can prove dangerous, stressful and even deadly as all the males hassle and attempt to breed with her. Too many males in a tank that is too small is a sure path to aggression and struggles for territory. Perhaps you are looking to breed your goldfish, or quite the opposite, you don’t want to end up with a tankful of baby fish! Maybe you’re just blessed with an incurable curiosity about the natural world. Whatever the case, read on to learn all about male vs. female goldfish and see what differences show up between the sexes.

Comparing Male vs. Female Goldfish

If you have a tank of goldfish and want to do a quick sexing of the lot, you could get a fish that you’re certain is female and put it in the tank with the others. The fish that tend to leave her alone are probably females, while those that chase and nudge her are likely males. However, this isn’t a necessary procedure once you know all the physical and behavioral differences to observe. So, let’s take a closer look for an education in sex differentiation.

Key Differences Between a Male vs. Female Goldfish

Some species of fish, such as clownfish, are actually able to change their sex. This sequential hermaphroditism, as it’s called, is not present in goldfish. However, goldfish only show sexual dimorphism, or gender differences, once they reach adulthood. For most goldfish, this occurs at around 9 to 12 months of age. While pet stores may occasionally tell you otherwise, short of a DNA test, there are really no reliable indicators of the sex of juvenile goldfish. If you want to be sure to have both genders in your tank, the only methods are to buy in bulk or to just wait and see how they develop. If you take home at least 6 goldfish, you can be relatively certain that at least one of them will be a different gender than the others.

In male vs. female goldfish, the key differences are their coloration and markings, their body and vent shape, fin length, and overall behavior, especially during breeding season when the males begin to chase the females to initiate spawning.

Male vs. Female Goldfish: Size

There is not a noticeable difference in male and female goldfish in terms of size. In a pond, goldfish can grow to be 2 to 10 inches on average and weigh around 5 to 9 pounds. They may grow even larger in the wild, up to 14 inches, with adequate space and resources. As pet fish in home tanks, they usually grow to be 1 to 2 inches long and hardly ever exceed 6 inches.

Male vs. Female Goldfish: Coloration & Markings

Goldfish are named for their iconic golden-orange hue but they actually come in a variety of other colors as well, such as red, blue-gray, brown, yellow, black and white. The shiny glowing orange hue is the most common color. This coloration will tend to show up brighter and bolder in females than in males. If looking to determine sex by color, compare the hues of nearby fish to one another and decide if any stand out, these are more likely to be females. Fish that are more pale or dull in color than others may be males.

Males may develop “breeding stars”, or tubercles, which are small white spots on their gill-shields that are triggered by testosterone and appear in an organized pattern. These may serve to protect against injury, indicate dominance, or advertise health to a potential mate. Tubercles may only last during the breeding season, but older males may display these spots all year long. They’re normally present on the pectoral fins, face and the scales. This is not a reliable method of identification, since they may or may not be present. But if you see tubercles on your goldfish, you can be relatively certain that it’s a male. 

A note of caution

If you have already determined that your fish is female by other means and notice that she has small white bumps that look like tubercles, it could be a case of the ich parasite. This is a fairly easy condition to treat, but can be deadly if left to progress, so be sure to initiate treatment as soon as possible.

The midline ridge will present differently in male and female goldfish. This is a raised line that runs from the pelvic fins to the vent. It will be more obvious in males, and in females likely very faint or even non-existent.

Male vs. Female Goldfish: Body Shape

Female goldfish tend to have rounder and thicker bodies than their male counterparts. They are also deeper-bodied, which means that from a side view they are longer from back to belly. During breeding season, female goldfish will develop eggs, or roe, within their bellies. This will cause a bulge to grow on one side, giving them an asymmetrical look. Males usually have thinner bodies that are more streamlined than female bodies.

Male vs. Female Goldfish: Vent Shape

At the base of a goldfish belly there is a vent, or anal opening. Female goldfish tend to have a rounder, more bulbous vent than males that will protrude slightly as the season for breeding approaches. Females may also display anal fins that are a bit thicker than those of the males. A males vent is normally narrow and oval-like in shape. It will likely be concave rather than protruding out as the females does.

Male vs. Female Goldfish: Fin Length

Goldfish have pectoral fins, which are two fins on each side of their bodies behind their gills. Male pectoral fins are often longer and tend to be more pointed than females, which are shorter and rounder.

On the underside of your fish you’ll notice an anal fin. This is reliably longer and more pointed in males than in females, hers will be short and curved.

Male vs. Female Goldfish: Behavior

Overall, female goldfish tend to be more energetic than the males, who are usually less active. However, this changes during the breeding season. It is a very strong indicator that you have a male goldfish on your hands when you see it chasing another fish during spawning season. Males will chase females around the pond or tank that they’re in, remaining close behind her or slightly beneath, also sometimes nudging her. He will then attempt to spawn with her by pushing her against the tank or nearby plants, urging her to release her eggs. This spawning chase can run up to several hours, fully exhausting both fish. When she finally does release her eggs, there will be up to 1,000 of them, which appear as white, yellow or orange little transparent bubbles. They will stick to plants or other nearby surfaces and then the male will release his milt, or sperm, to fertilize them.

Physical indicators in addition to behavioral cues are helpful, since males will also chase one another around to show dominance even when there are no females present.

Up Next

• Goldfish Gestation Period: How Long Are Goldfish Pregnant?
• Goldfish Lifespan: How Long Do Goldfish Live?
• The Healthiest and Best Goldfish Food for Your Pet

The photo featured at the top of this post is © iStock.com/satit_srihin