You can determine the sex of your fish by inspecting its gonads (reproductive organs), which are located towards the top of the gut cavity. Mature female fish will have orange ovaries and male fish will have white testes (see photos above).
Male and female fish can be distinguished by their reproductive organs, which are different in size, shape, number and colour. Male fish also have larger fins than females.
The internal reproductive organs of fish are called gonads. They contain eggs or sperm cells, and are located in the lower part of the body cavity just below where you can see your backbone. The female gonad is a mass of tissue that contains eggs; it is pinkish-red in color with no visible ovaries or openings for eggs to exit or enter. The male gonad is a mass of tissue that contains sperm cells; it is darker red in color with visible testicles (testes).
Gonads swell up when they mature into adults during sexual maturation, then begin to shrink again once spawning has occurred