The short answer
There’s no single rule — sexing depends entirely on the species, and some fish show no reliable outward difference at all. In general, look at colour, fin shape and body shape, and remember that these clues usually only appear once a fish is mature. Juveniles are notoriously hard to sex, so patience often helps more than a magnifying glass.
General clues
Across many species, males and females tend to differ in a few ways:
- Colour — males are often brighter and more boldly patterned, especially when displaying or breeding.
- Fins — males frequently have longer, larger or more elaborate fins.
- Body shape — females are often rounder and fuller-bodied, particularly when carrying eggs.
- Size — depending on species, one sex may grow noticeably larger.
None of these is universal, so always check the specific fish rather than assuming.
Species examples
A few common tank fish to show how varied it is:
- Guppies, platies, mollies, swordtails (livebearers) — easy: males are smaller, more colourful, with a modified anal fin (gonopodium); females are larger and plainer.
- Bettas — males have long, flowing fins and vivid colour; females are shorter-finned and often duller.
- Many tetras and rasboras — subtle: females tend to be rounder, but the difference is slight.
- Corydoras — females are usually broader and taller-bodied, best seen from above.
For details on any one fish, our species care sheets note sexing where it’s practical.
The bottom line
Start with the species, wait for maturity, and read colour, fins and body shape together. Browse specific guides like the guppy or betta care sheets, and see all fish in the species library.