How to Tell the Gender of a Fish

sick fish

How to tell the gender of a fish? There are some situations in fish keeping where you will need to know which are boys and which are girls. For example: trying to set up a single sex population can help keep spawning practices from hurting your fish, but determining males from females can sometimes be a little tricky. Depending on your fish species, there may be a few things you can look for to determine the difference. Unfortunately, some species have ZERO external characteristics, some you can only determine by subtle behavioral cues, and others… well, you might as well just flip a coin.

Keep in mind that reproductive function may be limited in fish not receiving appropriate nutrition or space. When resources are scarce, fish will forgo reproductive function in order to survive. Fish in situations that are being stunted due to space or dietary restrictions are unlikely to reproduce and therefore, show any external signs of gender differentiation.

Fish will reach reproductive maturity at different ages and various species will have different timelines. Provided they receive correct nutrition when they are very young and how much food the receive as juveniles will directly influence how quickly fish become mature enough to reproduce.

Common Pet Species: How to Tell the Gender of a Fish


Female goldfish usually have fat, round little bellies, full of eggs. Males show two distinct physical characteristics: 1) Roughened pectoral fins 2) raised bumps on their operculum (see picture below). This is most common in comet goldfish. Not all goldfish, especially fancy varieties, will show these sex differentials.


Some people say that goldfish characteristics will show on koi, but this is false. Males tend to be slightly smaller and skinnier throughout their bodies. Females will be larger and rounder. During spawning season, a female’s fish’s vent will be swollen and puffed out, compared to the male’s that is all tucked in. However, some of these external visual cues can be misleading! Fish who we’ve sworn were one sex were confirmed to be the opposite on ultrasound. Our office uses ultrasound to determine sex, however, we are unable to determine reproductively-immature individuals or stressed individuals who have sacrificed their reproductive capabilities.

how to tell the gender of a fish
In addition to severe ulceration, this female fish has a swollen vent.


As with many various animal species, males get all the pretty colors to help lure females to mate with. Male bettas are the usual “betta” you think of, with pretty colors and long, flowing fins. Female bettas will have shorter fins and brown in coloration. Male bettas are very territorial and need to be kept separate from other males. Under the proper conditions, multiple females can be kept together.


These live bearing fish can become a big problem if you cannot separate males and females! With a gestation period of approximately one month, one pregnant mama can quickly become a giant school! Most female guppies are larger with rounder bellies, but the best way to tell is by examining the anal fin. This is the fin that sits under the fish, just behind their anus. Male’s have long, pointed anal fins, whereas the females will have a shorter, triangular-shaped anal fin. Do not count on color to tell you the difference!

And that’s just a sampling of a few common pet fish species. Here is a good resource for more info on various fish species. All fish species have their own methods of telling males from females, but if you’re not a fish, you might not recognize them! If you are interested in breeding your fish, we recommend contacting a professional breeder for some tips.

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17 thoughts on “How to Tell the Gender of a Fish”

  1. Revonda Stephens

    I have a fishing pond outside and I have a goldfish that has a growth on its back what can I do for it and will my other go fishes get this to. I like to send a picture so you can see it more clearly of what it looks like let me know

    1. a woman in a lab coat smiling for the camera.
      Dr. Jessie Sanders

      Your veterinarian can perform an ultrasound to determine if your fish is full of eggs or excess fat. Baby fish are called fry.

  2. I have five fancy goldfish of varying colors and details and I’d be curious as to the gender. Only one of all five has no belly at all. The others, even the only one in there with the bulgy bug eyes, all have much more flowy fins and round bellies. I’ve assumed the rounder ones are females and the one poor sap with no fat is a skinny Vinny. Would I be correct?

    1. a woman in a lab coat smiling for the camera.
      Dr. Jessie Sanders

      Due to their anatomical features, it is harder to differentiate between sexes of fancy goldfish. An ultrasound would be able to determine the sex provided your fish are fully mature.

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