The best way to distinguish between male and female flowerhorn fish is to consider the combination of body size, nuchal hump size and reproductive organ appearance. Additionally, male flowerhorns often feature bolder colors than females. However, as these sexing criteria are not always accurate, some aquarists prefer to purchase proved breeders whose gender is known with certainty.
Size and Shape
Male flowerhorn fish exhibit faster growth rates than females do, and they reach larger sizes, too. Sometimes, males are up to 2 inches longer than females are. Despite being shorter than males, females usually have wider cross sections than males do. While both genders can have nuchal humps -- large fat-and-water-filled growths located above their heads -- males usually have larger nuchal humps than females do. However, the size of the nuchal humps varies with the fish's health and stress levels, which can make it difficult to distinguish a small, stressed male from a female who has a hump.
Flowerhorn colors vary greatly, and it is difficult to make broad generalizations. A single flowerhorn may display red, blue, green, yellow and black colors, or may be largely limited to gray, black and white. Nevertheless, males usually exhibit more intense colors than females do. Females, by contrast, often bear a small black stripe on their dorsal fins that males usually lack. Nevertheless, the colors of your flowerhorn fish may be influenced by myriad factors, including health, social factors or stress level -- so do not use color as the sole criterion for determining gender.
Male and female flowerhorn fish have differently shaped vents. A male has a V-shaped opening, a female's is U-shaped. As spawning time approaches, you may see small reproductive protruding from the vents of flowerhorns. Males have square-shaped reproductive organs, while females have triangular reproductive organs.
Observing the breeding behavior of flowerhorns is another way of determining gender. By watching closely, you will see females eject eggs into the water, and males releasing sperm. Males often behave more aggressively during breeding encounters. Flowerhorn fish are aggressive, so breeding attempts are often conducted while the fish remain separated from each other. Usually, breeders use a permeable plastic partition to separate the fish and allows the males' sperm to reach the eggs and fertilize them.
As long as no human error or deliberate misrepresentation occurs, purchasing proven breeders is the most reliable way to know the genders of your flowerhorn fish. However, mature flowerhorns are much more expensive than juveniles are. This is primarily because young flowerhorns all look similar, clad in basic gray and black. Only once they reach maturity do they display bright colors. Not all flowerhorns mature into spectacularly colored fish -- which means that purchasing young flowerhorns is something of a gamble. Avoiding this gamble means paying a premium for mature fish, whose ultimate coloration is already apparent.