Minimum Tank Size: 10 Gallon (Short-finned Molly), 20 Gallon (Sailfin Molly)
Care Level: Easy
Water Conditions: 6.5-8 and Moderately Hard to Very Hard (Short-finned Molly), 7.5-8.2 and Hard to Very Hard (Sailfin Molly)
Temperature: 72-82 F (25-28 C)
Maximum Size: 4 inches (10 cm) Short-finned Molly, 6 inches (15 cm) Sailfin Molly
Mollies are one of those most popular fish in the aquarium hobby, and most people have had at one time, at least one of these gentle and attractive live bearers in their fish tank. Most people mistakenly believe that there is only one species of molly available in fish stores, but there are actually three separate species available in the aquarium trade. However, at this point, most of the mollies available are so hopelessly hybridized, that it would be impossible to determine which species they originated from.
If you ever come across wild mollies in a fish stores, they will usually come from one of two species. The first species is sailfin mollies (Poecilia latipinna) which are found in the coastal waters of the southeast United States and around the Gulf of Mexico. These are among the most common type of molly, and are incredibly prolific breeders in the wild. Because they live in coastal regions, they have a high tolerance for brackish water, and have even been found living in the ocean.
The other species that is commonly found in the aquarium trade is the short-finned molly (Poecilia sphenops). These fish are found in Mexico, Central America and in the northern parts of South America. The short-finned molly adapt most readily to the home aquarium, and is one of the easiest beginner fish available . Like their sailfin molly relatives, they can also tolerate high salinity in water, and do well in both brackish and freshwater tanks.
There is a third species of mollies that is occasionally found in the aquarium hobby, but it is far rarer than either the short-finned molly or the sailfin molly. It is the Mexican sailfin molly (P. velifera), and is very difficult to breed and keep in aquariums, though it does well in outdoor ponds. It is found in the wild in the coastal regions of Yucatan in Mexcio, and like the other two species, tolerates a very high level of salinity in the water.
Housing Molly Fish
The short-finned molly is undemanding when it comes to providing them with an aquarium, and most will happily live in a 10 gallon aquarium. But if you really want them to thrive, they should be provided with at least a 29 gallon long tank.
Sailfin mollies on the other hand, grow much larger than their short-finned cousins, and require a minimum tank size of at least 29 gallons. But like many other large fish in the hobby, the fish will do much better over the long run if they are provided with a large tank right off the bat. Larger tanks also have the benefit of offering a more stable environment for the fish, since they aren’t as prone to the sudden water quality fluctuations that smaller tanks suffer from.
All of the species of mollies that are commercially available are docile fish, and will do well in any peaceful community tank. And emphasis should be placed on the word peaceful – if there are any aggressive fish in the tank, the wide-bodied and passive mollies will be bullied mercilessly.
When it comes to filtration, mollies do much better when the tank is over-filtered. They aren’t necessarily a messy fish, but some of the hybrids are more susceptible to disease than some of the other beginner fish, and do better when living in well maintained water. The best filters to use are either a high quality hang-on-back filter, or a canister filter if you can afford the hefty price tag that comes with it (Click here for HOB filter reviews).
One question that comes up often with mollies, is “do mollies need salt in their water?” The answer to this question isn’t simple, and a lot depends on what species of molly you have. As with most fish, mollies do benefit with some aquarium salt in the water, but nothing has conclusively shown that mollies need to be kept in water with salt added. In fact, most mollies will thrive in a completely fresh water aquarium, with absolutely no salt added.
The myth that mollies have to have a large amount of salt in the aquarium likely stemmed from some of the hybrids being more sickly than other types of fish. Like any fish that has been over-bred (neons come to mind), the genetic line can sometimes be weakened, and the fish become more susceptible to diseases. Many people then falsely attribute the illness to a lack of salt, and a myth is born.
I have kept mollies for close to a decade now, and they have never been in a tank with any salt added. Aside from one minor disease outbreak a few years ago (from an improperly quarantined fish), they have always been healthy and have had no problems breeding in large numbers. In fact, many of the mollies that I currently keep, are descendants of the original mollies.
Feeding Molly Fish
In the wild, mollies are omnivores and feed on small invertebrates, algae and plant matter. In the home aquarium, they should be fed a high quality flake food, and a variety of vegetables. Their favorite vegetables are blanched zucchini medallions, cucumber medallions and shelled peas. If you don’t have the time to cut and blanch vegetables, then a spirulina based pellet or flake can take the place of plant matter in their diet.
In order to get them into breeding condition, they should also be fed live or frozen foods as a treat. Their favorite frozen foods are bloodworms, daphnia, and brine shrimp. They will also greedily accept any live foods that you can find to feed to them, with live blackworms, bloodworms, daphnia and brine shrimp being special favorites.
Breeding Molly Fish
The molly fish is a live-bearer, which means that its eggs are internally gestated until highly developed fry are born. Because it’s doesn’t scatter its eggs, there is usually a high survival rate among its fry (as long as the tank doesn’t have any large predators).
Like most other live-bearers, breeding mollies is incredibly easy. It really boils down to fish + water = hordes of babies. But as with other easy to keep live-bearers like guppies, you have to be careful what ratio of sexes you keep in the aquarium.
In any aquarium that contains both male and female mollies, you should try to maintain a ration of three females to every male. Male mollies will relentlessly mate with any females in the tank, and if their attention isn’t divided between several females, then the female they focus on will eventually become highly stressed and may even die from the constant mating attempts.
If mollies are in a community tank, then the best way to ensure some of their fry survive is to add some floating plants or some dense plants like Java moss. While the babies are born highly developed, their small size will still allow for larger fish to eat them. The more plants that an aquarium has, the better chance that some fry will reach adulthood. Floating plants are the best choice, as fry can easily hide from adult fish in the dense vegetation near the surface.