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The Goldfish Gazette, Issue #069 -- How to Sex Goldfish
September 28, 2019

Goldfish Care Tips

A Free Monthly Resource For Goldfish Enthusiasts
September 2019
Issue #069

In This Issue
How to Sex Goldfish (out of breeding season)

Goldfish are able to breed at 12 months of age.
Male/female differences are too subtle to detect under 6 moths old, but as they get closer to their first breeding season, differences start to show.

How to Sex Goldfish (out of breeding season)

I have been asked a number of times recently how to sex Goldfish. I assume this is either for naming reasons, or the owner is considering breeding them.

The ideal way to choose Goldfish for breeding is to raise 6-12 juvenile fish together, and once the spawning season arrives, pick out those fish showing obvious signs of what sex they are.

This isn’t helpful if you haven’t got the room to grow that many fish or, there aren’t that number of fish available.

Under 6 months Old

Fish under 6 months old are virtually impossible to sex.

Having said that I have had fish show breeding tubercles at 6 months of age.

This was useful, but could those not showing tubercles be females? Possibly, but I couldn’t be certain, as some males never show tubercles and rarely, some females do show tubercles.

Another indication of sex at this age is size. If you can be certain that all the fry from a spawning have been fed ample food from the moment they first starting eating, then females can often be larger than their male siblings, but that isn’t a guaranteed indication of gender either.

6-12 Months

At this age subtle changes start to take place. The leading few rays of the pectoral fins start to lengthen in males, finishing in a much sharper point than the females which remain rounded. This is more obvious in the 9-12 month period.

Body shape doesn’t help as males can have just as deep or deeper bodies than females in fancy varieties.

Adult fish

Once Goldfish have reach maturity, and especially if they have bred before, it can be relatively easy to determine the gender.

If a male has bred before, he often shows residual indications of breeding tubercles, most noticeably on the front rays of the pectoral fins.

Females of long bodied varieties are often heavier in the body compared to males of the same age, whether it is the spawning season or not.

If the differences are not obvious in fish you are wanting to sex, the next most obvious difference in the sexes is the anal pore or vent.

In males, from the side view, the shape is convex, with nothing protruding. The female is the opposite, with a concave appearance and her vent protruding slightly. This becomes more obvious closer to the breeding season.

From underneath the male vent is narrower and elongated, the female’s rounder.

Determining the gender of fish can still be a lottery. If there are a number of fish of the same age together, it is much easier to get a general feel for body shapes and if any are showing residual breeding stars or at least thicker front rays of their pectoral fins.

The last fish I purchased from a local specialist fish shop were a pair of Celestials.

Unfortunately, this shop feeds it’s Goldfish sparingly, if at all and these fish were extremely underweight. I was happy with the male as he had thicker leading pectoral fin rays and the correctly shaped vent.

The female was more difficult.

Even though these fish were long in the body, no fish in the aquarium had a deeper or thicker body shape.

I chose the female based on pectoral fin shape, and her very slightly protruding vent.

When I got the fish home, I still wasn’t convinced it was a female. It was only after a few weeks of heavy feeding that she developed a slightly deeper body than the male, and closer to the breeding season developed the usual asymmetrical (uneven) body shape typical of a female with ripening eggs.


Fish under 6 months old are virtually impossible to sex.

Fish between 6 and 12 months old start to develop subtle physical differences, but you can easily get it wrong.

Adult fish can be relatively easy to sex if they have spawned before, and have been kept in reasonable conditions and not starved.

Use more than one indicator to determine gender.

Comments? Ideas? Feedback? I'd love to hear from you. Just reply to this e-zine and tell me what you think, or what topics you want covered.

Next Month's Topic

The Nitrogen Cycle

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