Instructions
Compare the size of the fish in mature pairs or groups. The largest clownfish is almost certainly a female.
Observe the behavior of young fish. When juveniles are establishing which fish is top fish, the one that does most of the fin nipping and seems to "win" most of the fights is likely to be or to become the female. In a group of more than two, one fish emerges as the dominant fish and this one becomes a female. The next dominant fish becomes a breeding male. He is usually the second largest in the group. The others remain males but don't reproduce unless one of the dominant pair dies.
Examine the fishes' bodies. When a female is due to lay eggs, she becomes slightly swollen around the middle.
Watch the fish after egg laying. The clownfish caring for the eggs by wafting water over them with his fins is the male. The female is likely to take a more aggressive protective role by chasing away other fish.
How to Tell a Male From a Female Clownfish
When it comes to reproduction and gender, fish often show characteristics very different from those of mammals. Clownfish are a case in point. These colorful little fish are sequential hermaphrodites. They all start off as males and some become female, based on social cues. If you buy two juvenile clownfish, the socially dominant fish will become a female. Distinguishing between male and female in mature mated pairs is fairly straightforward.