Molly fish are small tropical fish that can come in a wide range of colours. They are popular fish for aquarium keepers of any level, and some of the most popular varieties include the Dalmatian molly fish and the balloon molly fish. If you have an interest in breeding molly fish, being able to distinguish between male and female molly fish is essential. Even if you don't want to breed them, being able to tell the difference between males and females is useful, and—once you know what to look for—it's easy to do.
Wait until the molly fish is about 2 months old. Molly fish live for 3 to 5 years, and you will start to notice secondary sex characteristics between the ages of 2 to 3 months.
Look at the anal fin. The anal fin on a molly fish is located on the belly of the fish; it's the fin closest to the tail. On a male, the anal fin is elongated and tubular in shape. This is because the anal fin on the male contains an organ that is meant to deposit sperm in the female. The female's anal fin is fan shaped.
Examine the body shape. The body of a female molly fish in profile is rounded than that of the male. The male tends to have a body that is slightly longer and thinner.
Inspect the colouration. In most types of molly fish, the males are more brightly coloured and showy than the females. That means that although both male and female might be orange, the male is a more vibrant hue.
Molly fish breed very easily. They bear live young, and because of this, their young are less dependent on the water conditions. If you have mixed tank of male and female molly fish, you can generally count on them producing young, also called fry. If you do not want to breed molly fish, make sure that the molly fish that you choose are all females or all males. Add a small amount of aquarium salt to the tank if you want to encourage your molly fish to breed.
If your molly fish end up producing fry, place the fry in another tank immediately. Not only will they be easy prey for the other fish in your tank, their parents may also eat them.