Mollies belong to the genus Poecilia. They formerly belonged to the genus Mollienisia, hence the common name mollies.
There are two species in the genus Poecilia that are most commonly known as mollies (Poecilia sphenops, Poecilia velifera). There is also a lot of color variation within each species. Most mollies are solid black in color. The mollies in the above photo are female marbled mollies. As can be seen in the photo below, male mollies have an enlarged dorsal fin and a modified anal fin called a gonopodium. Notice that the anal fin looks very different in the male and females.
Mollies reach an adult size of about 4 inches (10 cm) in length.
Mollies are peaceful fish and can be kept in a community tank. You should provide plants in their aquarium. It is also beneficial to add some aquarium salt to your molly tank. Because mollies like salt in their water don't keep them with other species that can't tolerate the extra salt. I usually use about 1 tablespoon of aquarium salt for every 5 gallons of water. Keep in mind that if you have a 10 gallon tank that your rocks, heater, and other items in your tank take up some space and so you may not actually have 10 gallons of water. Also, salt doesn't evaporate and so when you replace water in your tank don't add more salt to it. You can, however, add more salt when making water changes (as long as it doesn't exceed the recommended salt level). Some people recommend adding less salt than this for their mollies, but this works for me. Whatever you do, don't use iodized salt in your tank or it will kill your fish!
Mollies do best at a neutral to slighly alkaline pH and a water temperature of 77-83°F (25-28°C).
They are omnivores, eating both plant and meaty foods. They will readily eat flakes, freeze-dried, frozen, and live foods. You should try to include some plant matter into their diet.
Mollies are livebearing fish. As mentioned earlier, it is easy to sex mollies because the males and females look differently. It is best to keep at least 2 females per each male because livebearing males often harass the females. Plants are also good so that the female has plenty of hiding spaces.
Mollies are easy fish to breed in the aquarium. They engage in internal fertilization. The male uses his modified anal fin (gonopodium) to inseminate the female (just like guppies and swordtails). Once the eggs are fertilized the female molly carries them inside her while they are developing. Gestation takes around 60 days.
The female molly will then give birth to about 20-50 fully developed molly fry. If your molly parents are black then the fry will be solid black at birth. Adult mollies will eat their fry and so the fry must be separated from the adult fish until they are larger. Sometimes plants will camouflage baby fish. Hornwort is the best plant to camouflage baby fish. If you are trying to provide fry with good hiding places you should have hornwort planted in the tank and also as a floating plant. Although the hornwort provides some protection, I've often found that it is safer to keep the the fry separate from their parents.
You can use a breeding trap. You can buy these relatively cheaply at most pet stores. They are plastic containers that you put the pregnant female molly in (or any livebearing fish). The breeding trap floats in the aquarium. There are two compartments. The top compartment houses the pregnant female fish. There is a small space between the top and bottom compartment so that that the fry will drop through to the bottom compartment. It is possible for the fry to swim back through this space into the top compartment, but they usually don't. After the female has finished giving birth to the fry you should remove her from the trap and place her back in the aquarium. The plastic piece that separates her from the fry should be removed and you can raise the fry for a short time in the trap. However, the water in the trap tends to get somewhat stagnant and so this isn't an ideal place to raise the fry for long. If you do raise the fry in the trap you will need to clean out the trap and replace it with water from the aquarium on a regular basis.
Adult fish often become distressed when placed in the small confines of a breeding trap. If you are planning on using one, then try not to place the pregnant female in it until she is close to giving birth. You can usually tell when she is close because her abdominal area becomes larger as the fry develop.
If you don't want to upset the mother fish, then a separate tank to place the female molly in works best (as long as it has a lot of hornwort in it to prevent the mother fish from eating the fry). Because the mother fish is busy giving birth and there are no other fish present, this gives the fry a pretty good chance of survival. As soon as she is finished giving birth to the fry you should remove her and place her back in the main aquarium.
Feeding the Fry
Baby mollies can be fed newly hatched brine shrimp, newly hatched daphnia, or they can be fed powdered dry food for fry. I've successfully raised many baby mollies to adulthood using powdered fish food for fry, however, you will get better growth of your fry if you feed them live food as well as the dried food. Also see the article Feeding the Fry of Livebearing Fish.