How to tell males from females
The most-asked question in every aquarium group, asked most often about animals far too young to answer it. Each method below carries what it is actually worth — which differs by group — and the point at which it can first be read.
Why this page exists
Our own stocking advice tells the keepers of 59 species to hold a particular ratio of males to females, because spare males mean relentless chasing. That instruction is worthless to anyone who cannot tell them apart. This is the other half of it.
The gonopodium
Definitivefrom about 4 weeks oldLook at the anal fin, underneath and just behind the belly. In females it stays a broad triangular fan. In males it narrows into a rod and angles forward — that rod is the gonopodium, a rebuilt fin rather than an organ that appears from nowhere.
Where people go wrong
The danger is not that you will be fooled for long — it is that they are faster than you. The gonopodium starts forming at three to four weeks and is readable by about four, but they are sexually active by six. A tank bought as fry is very often already mixed and already mated before anyone thinks to separate anybody. If a female is carrying, she met a male long before you looked.
Reliability by group
Saddle and berry
Strongat 12–18 mm body length — under 10 mm nothing showsThe ovaries show through the shell behind the head as a pale yellow or green patch shaped like a saddle. Those are unfertilised eggs, and their appearance is what tells you she is mature. Once she is carrying, they move under the tail and are fanned by her swimmerets.
Where people go wrong
Juveniles under about 10 mm cannot be sexed at all — the saddle is not there yet and the body shape has not diverged. Buying ten and hoping for a mix is the normal way to start a colony, and there is no shortcut. The saddle is also easy on pale or translucent shrimp and nearly invisible on deep red or black ones, where you are left with body curve alone.
Reliability by group
Dorsal fin shape
Strongfrom about 4 monthsIn most gouramis the dorsal fin ends differently between the sexes, and it can be read from outside the glass without touching the fish.
Where people go wrong
Fails completely on fancy long-finned strains, where the fin has been bred out of its natural shape. If you are looking at a betta with flowing fins, this tells you nothing — use the ovipositor instead.
Reliability by group
The ovipositor
Strongfrom about 4 monthsA small white tube at the vent, just behind the ventral fins, visible on a mature female from below or head-on. It reads as a single white dot.
Where people go wrong
This is the method that still works on short-finned bettas and plakats, where finnage and colour have been bred into meaninglessness. It needs a good look from the right angle, so it is easier on a fish in a bag or a photographing cup than in a planted tank.
Reliability by group
Venting
Strongonly on a mature fish, easiest when she is in breeding conditionThe method breeders use when the answer has to be right. Hold the fish upside down in a wet hand or soft net and look at the two openings ahead of the anal fin. The forward one is the anus; the one behind it is what you are reading.
Where people go wrong
It is not equally good everywhere, and that is the part usually left out. On Rift Lake fish it is the most accurate method there is; on most New World cichlids it is not reliable, and angelfish cannot be vented at all. Handle with wet hands only, keep the fish out of water for well under thirty seconds, and do not check the same fish more than about three times. If two fish look identical, they are far more likely both immature than the same sex.
Reliability by group
Width from above
Strongfrom about 8 months, once they are adult-sizedCorydoras are sexed from directly above, never from the side. Look down at the group while they rest on the sand.
Where people go wrong
A well-fed male looks broad and a hungry female looks narrow. This is a comparison across a group, not a measurement of one fish — judging a single corydoras against a remembered shape is exactly how people get it wrong.
Reliability by group
Egg spots
A hint onlyfrom about 6 monthsThe yellow or orange ocelli on the anal fin of Malawi cichlids, traditionally explained as a lure that draws the female into mouthing them, fertilising the eggs she is already holding.
Where people go wrong
The most over-trusted trait in the hobby. Females of many species carry egg spots too, and plenty of males carry none — counting them is not sexing. Use it to form a suspicion and then vent the fish if the answer matters.
Reliability by group
Extended fins
Strongfrom about 8 months, and later in a suppressed maleOn many cichlids and killifish the male's dorsal and anal fins draw out into trailing points as he matures, while the female's stay rounded.
Where people go wrong
It arrives late, and a subordinate male living under a dominant one may never develop it at all. Extensions present are good evidence; extensions absent prove nothing whatsoever.
Reliability by group
Bristles and cheek spines
Strongfrom about a yearMale plecos grow odontodes — spiny growths on the cheeks and along the leading pectoral rays. Bristlenose males additionally grow branching tentacles across the snout and over the top of the head.
Where people go wrong
It takes a year or more, and for that entire year a young male is indistinguishable from a female. Bristles on the snout rim alone do not make a male — it is branching, and coverage onto the head, that separates them.
Reliability by group
Belly depth
A hint onlyonly on well-fed adults, and only as a comparisonFor most tetras, barbs and rasboras this is all there is: a conditioned female is deeper through the belly than a male of the same length.
Where people go wrong
Be honest about what this is — a comparison inside one well-fed group, and nothing more. A single neon tetra in a shop tank cannot be sexed, and anybody who tells you otherwise is guessing confidently.
Reliability by group
Colour and pattern
Strongfrom about 6 months, and only on a settled fishIn rainbowfish, killifish and several barbs the male carries the colour — deeper and more saturated, intensifying further when he displays.
Where people go wrong
Colour answers to mood, diet, water and lighting as much as to sex, and a shop tank shows every fish at its worst. A pale male in a bag is not a female — this is the single most common way rainbowfish get bought as a group of one sex.
Reliability by group
Adult size
A hint onlyonly across animals of the same ageIn many cichlids, catfish and loaches one sex simply grows larger — usually the male, though there are well-known reversals.
Where people go wrong
Worthless on fish of different ages, which is very nearly always the situation in a home tank. It is only worth reading across siblings raised together.
Reliability by group
Breeding tubercles
Strongonly in breeding condition — they vanish out of seasonGoldfish and their cyprinid relatives grow small white bumps on the gill covers and leading pectoral rays when they come into breeding condition.
Where people go wrong
Entirely seasonal: out of condition they disappear and the fish cannot be sexed this way at all. They are also easy to confuse with early whitespot, which is the same size and colour — the difference is that tubercles sit only on gill plates and fin rays, while whitespot scatters over the whole body.
Reliability by group
When the honest answer is that you cannot tell
Some of what people keep cannot be sexed by looking, and saying so is more useful than inventing a trait. Most aquarium snails are hermaphrodite, so the question has no answer rather than a hard one. Crayfish, crabs and most oddballs give themselves away only in breeding condition, if at all.
Sources
Silhouettes from PhyloPic, most of them public domain. The ones that require credit: Caridina multidentata — Douglas Teles da Rosa (CC BY 4.0) · Ancistrus martini — Grupo de Ictiología de la Universidad de Antioquia (CC BY 4.0)
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