It’s not easy to tell the sexes apart in goldfish. The males don’t get into fights and the females don’t have markedly different colours like a lot of other animals, so it requires a bit of care and looking closely to find out which goldfish is male and which is female.

Breeding Signs

The best time to tell male goldfish from female goldfish is the breeding season i.e. Spring. This is when the male goldfish will develop small white pimples (sometimes called tubercles) on the outside of the gill flap, and along the front edge of their pectoral fins (these are the ones just below the gills).

Also at this time of year the female goldfish will get a bit fatter because she is full of goldfish eggs (properly called roe). However, please don’t assume that a heavier looking body means a goldfish can be called Mummy. It could be a symptom of illness or disease, so look out for other signs of trouble.

The Vent

The vent is the opening at the rear end of every goldfish that is used for defecating and reproduction. In female goldfish it is a round-ish shape and is slightly raised above the level of the skin. The male goldfish’s vent is usually more oval in shape and does not protrude as much.

Be aware however that like most methods of sexing goldfish, comparing vents cannot be done at a glance. A bit of experience and careful examination is required.

Midline Ridge

Male goldfish often have a ridge that runs along the back half of their underside ending at the vent. This feature is usually absent in female goldfish, but can be present and should only be used as confirmation of other observations.

Chasing

In the breeding season male goldfish chase female goldfish. Out of the breeding season male goldfish chase male goldfish. Therefore to use this method of sexing it is vital to have closely observed the goldfish for a period of time before making any decisions.

Other

With experience and careful observation it is possible to learn how to tell male and female goldfish apart by the thickness of their anal fins, the shape of their pectoral fins and the firmness of their abdomens. Some people have even claimed to be able to tell them apart by colour. However, these methods can only be used with any degree of confidence after years of experience of keeping goldfish.

Summary

Wait until goldfish are mature before attempting to find out what sex they are. It is very hard to tell outside the breeding season without a great deal of experience. However, taken together the signs detailed above can lead to a definitive answer.

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