How to tell the sex of a fish: species-by-species guide

Livebearers: guppies, mollies, platies, swordtails

The easiest group to sex in the entire hobby. Livebearers have a clear anatomical difference that is visible on any fish old enough to have matured:

The gonopodium — the definitive male indicator

Male livebearers have a gonopodium — a modified, elongated, narrow anal fin used as an intromittent organ for fertilisation. It points forward and is distinctly rod-like. Females have a fan-shaped, triangular anal fin. This difference is unambiguous and visible without a magnifying glass in mature fish.

• Guppies — additional signs: Males are more colourful (often dramatically so — vivid tails, colour patterns), smaller than females, and have a larger, more elaborate caudal (tail) fin.
• Platies and mollies: Gonopodium distinguishes males. Females are generally larger and wider. Gravid females (carrying fry) develop a dark gravid spot near the rear of the abdomen.
• Swordtails: Males develop the characteristic sword-shaped extension of the lower caudal fin. This develops at sexual maturity — juveniles don’t have it yet.

Bettas (Betta splendens)

Male bettas are distinctive and generally easy to identify:

• Fins: Males have significantly longer and more elaborate fins — particularly the caudal (tail), anal, and dorsal fins. Long-fin varieties are especially obvious. Wild-type and short-fin (plakat) bettas have shorter fins but males are still clearly larger-finned than females.
• Colour: Males are generally more vivid. Females can be colourful but are typically less intensely pigmented. Under stress or in poor condition, both sexes can appear dull.
• Body shape: Males have a broader, more angular head. Females have a more streamlined, narrower body profile.
• Ovipositor: A small, visible white dot between the ventral fins, visible when viewing the fish from below — this is present in females. Males don’t have it. Easiest to see when the fish is resting on the substrate or a leaf.
• Bubble nests: Only males build bubble nests at the surface — a cluster of bubbles, often incorporating plant material. If your betta is building bubble nests, it’s male.

Tetras and rasboras

Most tetras and rasboras are among the harder fish to sex reliably. There is no equivalent of the gonopodium. The differences are subtle:

• Body fullness: Females are typically rounder and wider in the belly, particularly when gravid. This is most obvious when viewed from above — a gravid female tetra looks distinctly wider than a male of the same age and size. The difference is subtle in non-breeding condition.
• Size: Females are often slightly larger in many tetra species.
• Colour intensity: In some species (e.g., cardinal tetras), males may show slightly more intense iridescence, but this is not reliable across all species or all conditions.
• Gravid spot: In some tetra species, a dark gravid spot is visible in the lower abdomen of ripe females. Not present in all species.

Honest assessment: in a school of neon or cardinal tetras, you may not be able to sex individuals with any certainty unless you observe them during spawning behaviour. This is normal — many tetras are sexually dimorphic only in breeding condition.

Goldfish (Carassius auratus)

Goldfish are notoriously difficult to sex outside of breeding season. Most sources saying they’re easy are referring to fish in breeding condition.

• Breeding tubercles (definitive male sign): During spawning condition — typically spring, as water warms — males develop small white raised bumps called breeding tubercles on their gill plates and along the leading edges of the pectoral fins. These feel slightly rough to the touch. This is the most reliable indicator.
• Body shape (breeding condition): Gravid females become visibly rounder and often slightly asymmetrical — the eggs shift the body contour. Viewed from above, a ripe female is noticeably wider than a male of similar age.
• Chase behaviour: Males persistently chase females during spawning season, nudging the female’s abdomen. If one goldfish is being chased relentlessly by others, it is almost certainly female.

Outside of breeding condition, goldfish are best described as very difficult to sex reliably. Vent sexing (see cichlids section) can be done but requires experience.

Corydoras catfish

Corydoras are sexable but require viewing from the correct angle:

• View from above: This is the most reliable method. Females are distinctly wider and rounder in the mid-section, particularly when gravid. The difference is like comparing a wider and a narrower oval from a bird’s-eye view.
• Size: Females are typically larger than males at maturity — this is one of the species groups where female-is-larger is a useful indicator.
• Ventral area: The underside of the female broadens significantly more than the male.

Juvenile corydoras are very difficult to sex. Wait until they are 6–9 months old and well-fed before attempting identification. The difference becomes obvious in mature, well-conditioned fish.

Gouramis

Most gourami species have clear sexual dimorphism:

• Dorsal fin shape: Male gouramis (pearl gourami, blue gourami, three-spot gourami) have a longer, more pointed dorsal fin. Females have a shorter, rounded dorsal fin. This is reliable across most of the group.
• Colour: Males are more intensely coloured, particularly during breeding condition. The orange-red throat and breast of a breeding male pearl gourami is dramatic. Females are more subdued.
• Dwarf gourami: Males are vivid red-orange-blue. Females are silver-grey with faint stripes. The colour difference is unmissable.

Cichlids — including rams, angelfish, and discus

Cichlids vary widely in how easily they can be sexed. Some are obvious; some require specialist knowledge.

German Blue Ram, Bolivian Ram

• GBR females: Have a pink/violet flush to the belly — particularly vivid in breeding condition. Males lack this belly colouration. Females also have a slight pink tinge to the belly area in standard condition.
• Fin shape: Male rams often show elongated first rays of the dorsal fin (the rays appear as pointed filaments). Females have a more even dorsal fin profile.
• Body shape: Breeding females are plumper in the abdomen.

Angelfish

The honest answer: angelfish are among the most difficult fish to sex visually, and most methods sold as reliable are not. The only genuinely reliable method is vent sexing — examining the breeding tubes (genital papilla) during spawning condition:

• The male’s breeding tube is narrow and pointed.
• The female’s breeding tube is broader and blunter.

This requires the fish to be in spawning condition and close observation. In practice, many aquarists discover the sex of their angelfish only when they observe a pair spawning — which is the most reliable confirmation of all. Methods based on the shape of the forehead profile, fin length, or body shape are not reliably accurate.

General cichlid vent sexing

Vent sexing — examining the genital papilla — is the most accurate method for many cichlid species and applies to other species too. The papilla sits between the anus and anal fin. In males it is small, narrow, and pointed. In females it is larger, rounder, and more prominent. This takes practice to do accurately and is best performed when fish are in breeding condition.

Loaches

Most loaches (kuhli, clown, yoyo, chain) are challenging to sex. The main indicators:

• Body fullness: Females are rounder, particularly in the mid-section. Breeding females are obviously fuller.
• Pectoral fin: Male clown loaches reportedly have a slight V-shape notch in the caudal fin compared to females’ more even edge — though this is subtle.
• Size: Females tend to be larger in most loach species.

For most hobbyists keeping loaches in community tanks, sexing is not critical unless breeding is the goal. Clown loaches breed very rarely in captivity and under specific conditions.

Freshwater shrimp

Shrimp are actually easier to sex than many fish once you know what to look for:

• Saddle: A yellowish or greenish oval marking visible through the transparent body on the upper back (behind the head) of female shrimp — this is the ovary. Called the “saddle” and visible in females who are not yet carrying eggs.
• Eggs: Gravid females carry eggs under the tail (under the abdomen, between the pleopods). Cherry shrimp eggs are visible as a cluster of coloured eggs being fanned by the female. This is unmistakeable.
• Body shape: Females have a curved, rounded underbelly to accommodate egg carrying. Males have a flatter, more streamlined underside.
• Size: Female cherry shrimp are typically larger and more intensely coloured than males of the same line. Male cherry shrimp are smaller and often paler.

Quick reference by species

Species	Best method	Difficulty
Guppies	Gonopodium + colour	Very easy
Mollies / platies	Gonopodium	Very easy
Bettas	Fin length + ovipositor	Easy
Dwarf gourami	Colour	Easy
Pearl / blue gourami	Dorsal fin shape	Easy–moderate
German Blue Ram	Belly colour (pink in ♀)	Moderate
Corydoras	Body width from above	Moderate
Cherry shrimp	Saddle / egg mass	Moderate
Goldfish	Breeding tubercles (seasonal)	Moderate–hard
Tetras / rasboras	Body fullness (breeding condition)	Hard
Loaches	Body fullness	Hard
Angelfish	Vent sexing / spawning observation	Very hard
Discus	Vent sexing or spawning	Very hard