Instructions
Let the fish grow older. When bettas are babies, even experienced owners have trouble telling them apart. As the fish get older, their bodies will change and they will begin displaying different behaviors based on their sex. The most obvious of these behaviors is aggression among males.
Look at the fish's color. Color is one of the most obvious indicators of a betta's sex. Male bettas have bright, vivid coloring, while females' coloring is more subdued.
Look at the fish's tail and fins. A male's fins can be up to three or four times as long as those of a female. However, this method isn't completely reliable unless you have other fish from the same spawn. A female betta from one spawn may look like a male of another.
Look for an egg spot. Female bettas have a small white spot behind the ventral fins. Sometimes, this spot isn't immediately apparent, so don't assume the fish is male if you don't see one.
Check for a "beard," a membrane beneath the gill plate cover. When the fish flares its gills, this membrane is apparent. On males, the beard is much more obvious than on females.
How to Tell If a Betta Is a Boy or a Girl
When it comes to betta fish, it can be difficult to tell males from females. Male bettas are by far the more common fish in pet stores because of their vivid coloring and long fins, but you'll see females, too. And sometimes, the fish is so small that you're not really sure what its sex is. Here are some guidelines to determine whether a betta fish is a boy or a girl.