Minimum Tank Size: 29 Gallons (110 Litres)
Care Level: Easy
Water Conditions: 7.0-8.5 pH and Hard Water
Temperature: 70-79 °F (21-26 °C)
Maximum Size: 5 inches (20 cm)
The black molly (Poecilia sphenops) is small live bearer fish, that ranks among the most popular fish in the aquarium hobby. It is native to both South America and Central America, and can be found from Venezuela in the south, to Mexico in the north. It is also found on several Caribbean islands, and in recent years has become established as an invasive species in Japan, parts of the United States and in scattered pockets in Eastern Europe.
In the wild they primarily inhabit freshwater streams, but they can also be found living in brackish coastal sea waters and swamps. There is a mistaken belief in the hobby that black mollies are a brackish water fish, and while they are highly adaptable to salt water conditions, they prefer a freshwater environment.
They remain a relatively small fish in the home aquarium, and males will grow to a maximum of 3 inches (7.5 cm), while the females can grow up to 5 inches (12.5 cm) in length. Most mollies will live around three years in captivity, though some well cared for mollies can live for as long as five years.
Black mollies can be sensitive to poor water conditions, so they should be provided with a spacious aquarium which allows for more stable water parameters. While the minimum tank size for a small group of mollies is 29 gallons (110 litres), that doesn’t take into account the vast quantity of fry that inevitably arrive. If you want a tank where the fry can be grown out to a reasonable size, the minimum tank size rises to 45 gallons (170 litres).
It’s important that any tank containing mollies maintains a very strict ratio of the sexes. There should be at least two females for every male, and many people recommend at least a three to one ratio. The reasons for this ratio is that males relentlessly attempt to mate with females, and by providing more females, you ensure that no one female receives all of the males attention. Should you keep a lower ratio, the females will quickly become stressed from the constant mating attempts, and will often fall ill and may even die.
It is generally recommended that any tank containing mollies be heavily planted. The adult mollies will appreciate the planted environment, but more importantly, the plants will help with the fry survival rate. Some easy to care for plants that you can add to a mollies tank are Amazon sword plants, hygrophila polysperma, Java ferns, Java moss, water sprite and water wisteria.
Even though numerous articles recommend adding salt to a mollies aquarium, this is unnecessary if they are kept in alkaline, hard water. There are numerous studies that show mollies will be just as healthy if kept in aquarium without salt, though there is some evidence that salt can be beneficial to them if they are not kept in their preferred water conditions.
When choosing a filter for mollies, the best choice is generally a hang on back filter. While a canister filter is usually a superior filter, it tends to be overkill with mollies, and the price tag is usually at least double that of a hang on back filter.
I strongly recommend choosing an Aquaclear Power Filter for a molly tank. This filter combines excellent filtration with a durable design, that will keep your tank sparkling clear for years to come. You can also read the Aquarium Tidings Aquaclear Filter Review here.
Mollies are omnivores, and feed on invertebrates, plant matter and detritus in the wild. This diet should be replicated as closely as possible in the home aquarium, and they should be fed a high quality flake food, along with regular feedings of live or frozen foods and vegetables.
Unlike many other fish, vegetables are absolutely essential to a mollies health, and they should be offered blanched zucchini medallions, shelled peas and cucumber medallions at least once a week. If you can’t offer vegetables, they should be fed Hikari Algae Wafers or other herbivore food on a regular basis.
Their favorite live foods are mosquito larvae (illegal to culture in some areas), bloodworms, blackworms, daphnia and brine shrimp. They have similar tastes in frozen foods, and they can be fed frozen blackworms, daphnia, bloodworms and blackworms.
Black mollies are easy to sex, and the females will be larger than the males and have a triangular anal fin. Meanwhile, the males will be smaller, and have a gonopodium, which is a modified anal fin that serves the purpose of hooking onto the female and depositing sperm.
Breeding will normally take care of itself in a molly tank, and as long as you ensure that you maintain at least a two to one ratio of females to males, you will soon have pregnant mollies. The pregnant mollies can be identified by a dark gravid spot near their anal fin, and a considerably plump stomach.
Black mollies are a live bearer fish, and unlike most other commonly kept fish, they don’t lay eggs. The fry develop inside of the female, and they are born fully formed and swimming. Because of this, they generally have a very high survival rate, though some adult fish may still eat them.
When they mate, the male impregnates the female with a bundle of sperm on his gonopodium, which is delivered to her vent. Once the female is pregnant, the gestation usually take about 7-8 weeks, at which point the female may deliver upwards of one hundred babies. On average, the female will deliver anywhere from 40-100 fry, though you can expect younger female mollies to be on the lower end of that scale.
If you want a high fry survival rate, then the female molly should be removed from the main tank prior to giving birth. While mollies aren’t as predatory as guppies or platies, the adults will still eat many of the fry. After the female has given birth, she should also be removed from the fry tank.
The fry don’t require any special food, and right from birth they can be fed ground up flake food. They will also accept the usual fry foods, and can be fed baby brine shrimp, microworms or one of the many available commercial fry foods.
Black mollies are a color morph, and unfortunately they tend to be badly inbred these days. Because of their inbreeding, they are susceptible to illness, and aren’t as hardy as they used to be.
When purchasing mollies, always carefully examine the tank in the fish store, and don’t purchase them if there is any trace of illness. While this is true of most fish, it is extra important for mollies since they can often be wiped out by a simple illness.