We will be starting a new series of articles on how to breed Flowerhorn.
We will begin with sexing them. It is nearly impossible to sex Flowerhorn less than 3 inches. The only techniques that can be used at that size is the males will grow slightly faster and will usually be the most dominant fish in the tank of fry. With that said, there will be exceptions to both. At around 3-4 inches males will start to develop a KOK (hump). This hump is made up of fluid and fatty tissue and will actually increase and decrease in size depending on the conditions. When spawning, it will slightly increase in size. Males will also grow longer streamers from the Dorsal and Anal fins. Males will grow to be larger than females, often significantly larger. Females will have a more plump and rounded belly. When they become egg laden, this will become even more apparent. When mature their sexual organs will become visible. Directly before the anal fin will be the oviduct in the female.
It will be a round tube (for laying eggs) that in a mature female often be 1/16-1/8 inch long and about the width of a spaghetti noodle. Males will have a much smaller urogenital tube that will also be at the very front of the anal fin. It will look like a very small penis, but is not intended to ever be inserted into the female. Instead it helps them direct the spray of sperm onto the eggs that the female lays. Usually you cannot see it, but at spawning, it does become visible. The male urogenital tube will be about the width of a straight pin.