Do not touch your goldfish, as this can cause them damage. You will not need to remove your fish from its tank to determine its sex.
It will be easier to sex your goldfish when it has reached maturity -- usually at age 3 to 4 months.
Goldfish make rewarding and low-maintenance pets. When keeping goldfish, it is important to know whether your fish are male or female to aid or prevent breeding or simply for the purpose of naming your pet. Determining the sex of your pet goldfish can be done easily and simply at home.
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Look at your goldfish. Boys will tend to have a longer, thinner body than girls.
Examine the fins on your goldfish. A boy's pectoral fins will be more prominent and angular than a girl's, with white bumps to the front of the fins.
Study the shape of your goldfish's body. The female's abdomen will have a softer or possibly lopsided appearance. The male's abdomen will be harder and more symmetrical.
Check your goldfish's vent. This can be found on the underside of the fish, just in front of the anal fin. A female goldfish will have a vent that is more noticeable.
Inspect the gills. Boys will have white pimples on the gill covering.