Instructions
Compare several fish at one time. The fish with the larger caudal, or tail fin, is male. The male will also possess a much larger dorsal, or back fin, as well as a more impressive ventral fin, which is located on the abdomen of the fish. The side, or pectoral fins, which are located near the gills, will also be larger in the male fish.
Observe the color of the fish. The fish with the brightest color is male.
Hold a mirror against the aquarium glass. The fish that charges the mirror and flares its gill covers is the male. Male Chinese fighting fish are highly territorial and will challenge each other on sight. The male fish believes that the image he sees is another male and prepares to challenge the intruder.
Look at the belly of the fish just prior to mating. The fish which displays a tiny tube in the area where you would expect to see the anus, is the female. The tube is an ovipositor and is protruding from the vent of the fish. The eggs will be laid through this tube.
A fish that builds a nest of bubbles on the surface of the water is male.
Watch the fish during mating. The fish that curls itself around the other is the male.
Continue to observe the mating. The fish that drops the eggs is the female, while the fish that collects the eggs and places them in the bubble nest, and is the male.
Chinese Fighting Fish: How to Tell If It's Male or Female
The striking Chinese or Siamese fighting fish is native to Asia. These small freshwater fish inhabit densely overgrown bodies of water and are able to breathe oxygen from the environment. Chinese fighting fish use a special organ on their head, called a labyrinth, for this purpose. Male fighting fish, particularly those which have been captive bred for color and overall shape, have far more impressive fins than the drab females. A difference in fin shape and size, as well as breeding behaviors, are used when sexing these fish.