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Boy or Girl Fish

Question: How do I tell the sex of my fish?

Answer: Depends on the fish.

Like everything else in fish keeping, there is no simple answer, and believe me, I' am not just trying to be clever. Some species are really easy with glaringly obvious differences between the sexes, e.g. the live bearers. With others its more subtle, like slightly longer fins, or more rounded bodies, or even small colour variations. Some however, have absolutely no differences what-so-ever, both sexes are identical.

To complicate matters even more, many fish are trans gender, (sequential hermaphrodites to use the proper terminology), they can change sex in response to external stimuli. In a group of all males, or all females, one or more will change into the opposite sex so that there is at least one pair. In some species this can happen in only a few days.

Live bearers: Perhaps the easiest category of fish to sex are the live bearing fish. While males are often smaller and more colourful the real give away is their external sexual Sexual differences in Live Bearing Fishesorgan, the gonopodium, which makes it easy to differentiate males from the females. The gonopodium is a modified anal fin which is used to fertilize eggs. In the male the anal fin is rod shaped, while the female has a traditional fan shaped anal fin.

Siamese Fighting Fish (Betta Splendans): Male fighters are the more colourful of the sexes, and have the long flowing fins. Females have short fins, and are their colours are more drab, although some are still quite colourful.

Barbs and Tetras: Some species from these families do exhibit colour variation between sexes, but for most its subtle differences in body shape. With Minnows, Neon's, Danios and such like, the females are more rounded while the males have slightly longer fins.

Cichlids: With such a vast family it would be impossible to list every variation, or combination for that matter. The best I can do is say that sometimes the males are slimmer, larger, have longer fins and may be more highly coloured as in this pair of Rams.

Pair of Rams highlighting the sexual differences

However two of the best known Cichlids, the Angelfish and the Discus have no sexual differences what-so-ever. When they are actually laying their eggs the female's breeding tube, (ovipositor), extends, only then, and only for a few hours is it possible to tell the difference.