How to Tell if Your Goldfish Is a Male or Female

Identifying Females

Look for a rounder, thicker body shape. Female goldfish tend to have a rounder, thicker body shape than males of the same age and species. They also tend to be deeper-bodied, which is useful for identifying sex from a side view. As breeding season approaches, the females will begin to develop roe (eggs), which may cause a bulge on one side, making the female look asymmetrical or lop-sided.

Look for a protruding vent. The vent (anal opening) of a female goldfish is rounder than the male's and tends to protrude from the body slightly as breeding season approaches. When viewed from the side, the vent may appear as a raised surface on the female's abdomen. In addition to a protruding vent, the female's anal fin may appear slightly thicker than the male's.

Identifying Males

Look for tubercles. One of the main tell-tale signs that your goldfish is male, is the development of tubercles (small white spots) on their gill-shields. Usually the breeding tubercles will only last during the fish's breeding period, however in older males who have lived through several breeding seasons, the tubercles may be present all year round. The tubercles may also appear on the pectoral fins, on the face and on the scales on the body of the fish. Be aware that although the presence of tubercles is a good indication that your goldfish is male, their absence does not necessarily mean that your fish is female, as not all males will develop tubercles.

Look for a thinner, more streamlined body shape. Males tend to have longer, thinner, more streamlined body shapes than females of the same age and species.

Look for a concave vent. A male goldfish's vent is usually narrow and elongated, making it somewhat ovular in shape. It will also be concave (an innie), rather than protruding.

Look for a midline ridge. If possible, look at the underbelly of the fish to see whether it has a midline ridge: a raised line running from the back of the pelvic fins to the vent. In females, this line will either be very faint or non-existent.

Watch for chasing behavior. One of the most reliable ways to identify male goldfish is to observe their behavior during spawning season. A male goldfish will chase a female around the tank or pond, staying close behind and slightly beneath her, sometimes nudging her rear. The male will also attempt to push the female against the side of the tank or into any plants, in order to force her to spawn. However, in the absence of females, male goldfish will also chase each other around, so it's a good idea to use physical indications as well as behavioral ones to identify sex.

Understanding Difficulties

Understand that gender differences will only appear once the goldfish reaches maturity. The differences between male and female goldfish will only become obvious once the fish reach maturity, which usually takes about a year. However, the rate of maturation will depend on species and sex. Some breeds of male goldfish will reach maturity by the time they are 9 months old, while some females may take up to three years to reach maturity. Short of DNA analysis, identifying the sex of young goldfish is practically impossible. If you wish to increase your chances of having both male and female fish, your best bet is to purchase at least six healthy fish of the same variety. Statistically, there is a 98% chance that at least one fish will be a different sex from the others.

Know that there is no foolproof method for sexing goldfish, other than observing the spawning process. Accurately identifying goldfish as either male or female is very difficult, and even the experts get it wrong from time to time. This is due to the fact that there are many exceptions to the rules: Some male goldfish will not develop tubercles, while in rare cases a female goldfish will. Some female goldfish will not have protruding vent, while male goldfish sometimes will. In addition, some breeds of goldfish do not comply with general rules - for example, some breeds (such as ranchu or ryukin) naturally have very round, thick body shapes, which makes gender identification through body shape almost impossible.Tell if Your Goldfish Is a Male or Female Step 9Bullet2.jpg As a result, it is best to identify the sex of your goldfish by observing multiple signs, rather than trusting a single sign alone.

Understand that these identification methods only hold true for healthy, well-fed goldfish. Unhealthy goldfish may not follow regular spawning behavior or develop the usual physical characteristics that indicate gender. As a result, it is important to ensure the health of your goldfish (this includes things such as providing good quality water and fish food) before you attempt to identify the sex. For example, an unhealthy male goldfish may not develop the tubercles usually seen during spawning season, while an unhealthy female may not have a convex vent. Body shape can also be misleading. A thin goldfish may be mistakenly identified as male (as males are usually smaller), but you may be looking at an underfed female. On the other hand, you may read a distended abdomen as a sign that the fish is female, but the swelling may be a sign of dropsy (an internal bacterial infection).