Most people will have seen a molly fish at one point or another. They are one of the most commonly kept fish amongst fish keeping enthusiasts.
They are perfect for people new to the hobby as they are easy to care for.
As an active and social group, you will see plenty of interesting behaviors and unique personalities begin to develop. They are a simple way to add some beautiful fish to your tank.
In this article we cover the most popular varieties as well as explaining their ideal setups, preferred diet and much more.
|Lifespan:||Up to 5 years|
|Size:||Up to 4.5 inches|
|Minimum Tank Size:||10 gallons|
|Tank Set-Up:||Freshwater, planted tank|
|Compatibility:||Peaceful community aquarium|
Molly fish, or just mollies, are freshwater fish from the genus Poecilia, in the Poeciliidae family. All the species in this genus are mollies except for the Endler’s livebearer.
There are currently 40 species in this genus, so 39 of them are mollies.
These fish are native to the Americas, but their habitat can vary significantly.
An interesting fact about mollies is that, like guppies, they are livebearers. This means that they hold their eggs inside their body until they can give birth to live young straight into the water.
They are a very popular group of fish. Most species are hardy and easy to care for, making them great for beginners. They work well in peaceful community aquariums. If kept in a healthy environment, a group of mollies will thrive and can live up to 5 years.
It’s rare that you will come across a fish store that doesn’t sell mollies – they are one of the most popular freshwater fish.
Common varieties can be found for $2-$4 per fish, though rarer species cost more.
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They are a peaceful bunch most of the time but can show signs of aggression when crowded or surrounded by aggressive tank mates. Therefore, it’s important that their tank is big enough, and that they have suitable tank mates (more on this later).
Mollies are active and social so they enjoy schooling together. A shoal should be predominantly female since males are known to harass and stress females.
It’s easy to spot personalities in active fish like these. Watching them for a while will quickly get you attached as you notice their individual differences.
Molly Fish Types and Appearances
There are lots of molly fish varieties – most are very similar to the common molly (Poecilia sphenops). The main differences are colors and patterns, but sizes and shapes can differ too.
The common molly has a flattened body. It’s tall in the middle and narrows towards a point at the mouth. At the other end is the caudal fin which is a large fan that can be transparent or colorful.
The dorsal fin can be raised as a fan, resembling the caudal fin, or flatter against the body.
Sexing these fish isn’t too hard and they are easy to mate. Females have an anal fin that spreads into a fan, whereas a male’s is pointier. Females are larger too, potentially reaching 4.5 inches. Males grow to 3 inches.
Pregnant females will look even larger. Their bellies swell to be much bigger than other females.
Here’s a summary of a few popular mollies for home aquariums:
Black Molly Fish
These fish are black all over. Apart from that, they’re virtually the same as the common molly.
This variety is bred in lots of different colors and patterns. It’s most distinctive feature is the large dorsal fin which is taller and runs from behind the head to just before the caudal fin.
Just like the sailfin molly, the lyretail stands out because of its fins, but this time it’s the caudal fin. The top and bottom of the fin narrow into points which trail behind the rest of the fish.
Other popular mollies include the Dalmatian, Balloon, Red, White and Orange.
Habitat and Tank Conditions
In the wild, a molly’s habitat can vary. They have adapted to lots of different water conditions, including a tolerance to brackish waters and high hydrogen sulfide levels.
They are mostly found in the shallow parts of rivers and streams across North and South America. Here the substrate would be sandy with rocks and debris sat on top.
Plants would be plentiful too, they mainly use these as shelter but they’re important for reproduction as well. The rivers are in tropical climates so get a good supply of light for plant growth.
The waters would be warm and slow moving. pH tends towards being slightly alkaline.
Mollies bred in captivity for the aquarium trade are all used to similar environments, so you don’t need to worry about each variety needing a different setup.
A layer of sandy substrate along the bottom of the tank is a good idea. Whilst they won’t spend much time down at these levels, the fine-grains are good for keeping plants.
You are free to choose your favorite plants, but taller options like Anubias nana make good shelter for these mid-level swimmers. Use decorations like rocks to create caves and crevices. They’re useful for all fish to get away from others that could be harassing them.
A tropical fish like this needs a heater in the tank to maintain temperatures of 72-78°F. Keep pH between 6.7 and 8.5, and hardness between 20-30 KH.
Some people recommend slightly brackish waters for your mollies. The benefits of this are not confirmed, and it limits the tank mates you can keep with them, so we would avoid this.
Standard aquarium lighting will be enough. You don’t need any other special equipment (like a water/air pump) either. These fish are used to slow-moving water which the filter outlet will provide.
What Size Aquarium Do They Need?
A molly fish needs at least a 10 gallon aquarium. This size would be suitable for up to four depending on the species – larger mollies like the sailfins will need a 30 gallon tank.
Each additional molly will need around 3 gallons to live comfortably.
As peaceful fish they don’t cause many problems, so they go great with other peaceful species in a community aquarium.
There are lots more to choose from, most small peaceful community fish will be fine.
Fish to avoid are large or aggressive. Large fish will try to eat your mollies, aggressive ones may attack and stress them out to the point of death.
Cichlids are a group that contains numerous bad examples. For instance, Convict Cichlids have an aggressive reputation and don’t play well with others. Some cichlids aren’t a problem though, like Angelfish.
Most invertebrates will be happy living alongside your mollies, they’ll mostly be ignored. Shrimp and snails are popular options, and there are plenty of species to choose from.
Keeping Molly Fish Together
You should keep them in groups of at least four or more, they naturally stick together. The group should be made of mostly females since males are known to harass them. Reproduction is usually the only cause of aggressive behavior.
What To Feed Them
As omnivores there are lots of different food types that you can feed them. In the wild they’d eat small invertebrates, but mainly plants and algae. There are many similar foods to give them in an aquarium.
Algae is a big part of their diet, they use their lips to scrape it from surfaces. Vegetation is important too, you can use small pieces of green vegetables from your kitchen. Lettuce, spinach, and zucchini are all good options to drop into your aquarium. You can even use them to make your own homemade fish foods.
Artificial foods like flakes and pellets are an easy option. Try to supplement these with other foods to provide an wide range of nutrients.
Live and frozen foods are excellent sources of protein. Bloodworms and brine shrimp will be happily accepted, but most other options work well too. Live foods are more interesting for the fish come feeding time too.
Feed them small amounts twice a day. This gives their digestive system chance to process the food. Give them as much as they can finish in two minutes.
Molly Fish Care Guide
Mollies are some of the most hardy and more adaptable fish out there.
Just like any other fish, the big thing is a clean tank. You need to know how to clean effectively. This includes monitoring the water conditions and checking that ammonia and nitrites are at 0ppm.
Your mollies may develop a condition called “molly disease” (or “livebearer disease”, or “shimmies”). This isn’t actually a disease because it’s caused by poor water conditions.
Affected fish will swim around less, wiggling and shimmying in one spot. Check your water parameters and correct any issues, once conditions return to normal you should see improvements in the health of your mollies.
Common signs include a loss of appetite, inactivity, spots, wounds and color changes.
Cleaning the tank and switching up their diet are a couple of easy ways to reduce the intensity of disease. If the problems persist, there are treatments that you can purchase from stores.
How To Breed Molly Fish
Mollies are livebearers, which means their eggs develop inside their body so that live fry can be released. They are one of the easiest fish groups to breed in captivity and will mate regularly.
Conditions in a breeding tank need to be perfect, the water (and tank in general) should be clean.
Raising the temperature slightly could help to initiate mating, but don’t go higher than 78°F. Males perform a courting display for the females, when the female is ready to mate she will allow the male to fertilize her eggs. Sometimes the male tries to “sneak-copulate” where he approaches an unaware female from behind.
Females often choose to mate with the largest males.
After fertilization it will take 35-45 days before the young are released. Larger females could release up to 100 juveniles.
The young need to be separated from the adults or they will get eaten. One option is to put pregnant mollies in a breeders box before they give birth, the young are free to leave the box through small holes but the adults stay trapped.
You can feed them foods like broken flakes until they’re big enough to eat the same foods as the adults. At this point they can be mixed back in with them.
FAQs on Breeding
Q: What does a pregnant Molly Fish look like?
A: A female’s belly will swell to be much larger than usual. This is generally easy to spot, especially when comparing them to other mollies in the tank.
Q: What do Molly Fish babies look like and how do you care for them?
A: The fry look like small versions of adults, with large black eyes. Colors will vary depending on the species. Keep the fry separated from other fish (including their parents) until they are a similar size.
Are Molly Fish Suitable For Your Aquarium? (Summary)
There aren’t many reasons not to get mollies, they are a great choice for community aquariums. You just need to consider do you have the right tank mates? You need to avoid large or aggressive fish.
Their health is also dependent on the conditions of the tank, which need to be within their preferred range. They can tolerate a wide range though, so they can go in lots of different tanks.
You won’t have a problem with their diet either, they can eat lots of different food types.
Overall, they are very easy to care for if you have the right tank. In return you get a group of fish that are easy to breed, introduce lots of color, and display lots of activity.
What is your favorite kind of Molly Fish? Let us know in the comments below…