How To Determine The Sex Of Your Goldfish

You may be trying to determine the gender of your goldfish for breeding purposes, or because you want to pick an appropriate name.

Male and female goldfish also behave in very different ways, which could be important if you want to avoid any in-fighting or breeding behaviour. But this makes this one of the easiest methods in which to tell their gender if you observe them for a few hours.

Never just observe one trait about your goldfish to determine the sex, however. There are a few other signs you should look out for too, and painting an overall picture is the safest method.

Most of the signs will appear in spawning season, usually between April to August after the colder weather, which gives you a generous time frame for observation.

Females

  • Body Shape

Females are often slightly rounder and thicker than their male counterparts when they are at the same age and from the same subspecies.

This is because they are the carriers of the roe (eggs) during the breeding season. When this is in full flow, the roe may cause a bulge on one side, making the fish look uneven and swollen.

  • Protruding Vent

The vent (anal opening) of a female goldfish is often a lot more pronounced than that of a male, especially around breeding season. Their anal fin, which is between their tail and pelvic fin, may also be larger throughout the year.

Males

  • Breeding Tubercles

A tubercle is a small white spot near gill shields. Again, they are often most pronounced in the breeding season, but in older males who have seen quite a few breeding seasons, they may appear throughout the year. They can also appear on the fins and scales of the fish. Not all males will have these however, so if a goldfish doesn’t have them, it doesn’t necessarily mean they are female.

  • Midline Ridge

If you are able to look at the underside of your goldfish, they may have a raised line running from their pelvic fins up to their vent. In females, this line is often barely visible.

  • Chasing

Seen one fish chase another? During spawning season, a male fish may closely follow a female and occasionally nudge her. This may get quite forceful, pushing her to the side of the tank or into decorations to force her to spawn. Males can also chase other males if there is no female presence, however.

Determine Sex Of Goldfish

When Do These Differences Appear?

They will most likely only show when the goldfish is at the age of maturity. This is often around a year old on average but could be between nine months and three years depending on the particular breed.

Identifying goldfish is easiest during the breeding season. Even if this begins at a year old in your particular species, it can still only really become regular when they reach the age of three.

Don’t know how old your fish is? For every year of their life, they develop a ring on their scales. A bit like a tree really, except you’d need a microscope to see them.

Even Professionals Get It Wrong

As mentioned above, some male goldfish can have tubercles but some may not develop them at all. There could also be differences in behaviour depending on the general way your fish acts – they do have personalities, especially if living with other fish.

Chasing may be a sign of intimidation, especially if they are nipping the tails of the other tank mates. This can be confusing as seeing their breeding chase can also be quite violent-looking. In this case, fish may need to be separated.

Some goldfish breeds can also have naturally round, large bodies. The Oranda, Pearlscale or Lionchu are very ‘bumpy’ compared to the Fantail or Common Goldfish for example. You may also think you have a male but are just looking at an underfed female.

This is why it is important to observe all possible signs.