Male vs. Female Fish

How can you tell male and female fish apart? 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.

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.

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.


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.

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.

A colorful, male betta


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. All fish species have their own methods of telling males from females, but if you’re not a fish, you might not recognize them!

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