How to tell a male koi from a female koi?
It is much easier to determine the gender of koi than it is for a goldfish. With that said, it is almost impossible to visually tell the difference betwen male and female koi if they are immature in age and size.
As koi grow, prominent signs of their gender develop. Male koi will have more pointed fins, while female koi have more rounded fins. Also, male koi have a more slender appearance, while female koi have a more oval body shape. Female koi fish will be full of eggs during spawning season, giving them a more blimp like appearance.