How to Tell the Difference Between a Boy Fish & a Girl Fish

The goldfish is a common pet and relatively easy to sex.
The goldfish is a common pet and relatively easy to sex. (Image: Jupiterimages/ Images)

"Sexing" is the correct term for determining whether a fish is male or female. Some people, especially in agricultural circles, even make a career out of sexing various animals. When it comes to fish, determining the sex can be particularly difficult because male and female genital regions are similar, and you'll need a keen eye to determine a difference. Although the exact process of sexing a fish will differ according to the type of fish, a few basic principles remain the same.

Things You'll Need

  • Jewelers loupe
  • Light source

Purchase a jewellers loup. These are powerful magnifying devices that you can hold in front of one eye.

Look at the size of the fish. This is the quickest way to determine the sex of the fish, but it is also not entirely accurate. Usually, with species such as goldfish and koi, the female is larger and plumper than the male. Males are more torpedo-shaped.

Look at the pectoral fins just below the fish's gills. Male goldfish have longer and larger pectoral fins and females have rounder and shorter ones.

Pick up your fish gently and turn it over. Using the jewelers loupe and a light source, examine the underbelly of your fish. Look at the central region between the head and the anal fin. You should be able to identify the anus as a small circular area. Directly below it is the genitalia, which is also known as the vent. By examining the vent, you should be able to determine the sex.

Look at the size of the vent. Males generally have smaller, more rounded vent areas, while females have wider vent regions. In some species, the vent area is almost identical, and looking at the color will help. Compare it to photos of the vent area of other fish of the same species. Websites such as Exotic Tropicals has a number of pictures you can use.

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