Molly fish are one of your classic species of freshwater fish. They’re popular because they’re low-maintenance care, straightforward for beginners, and come in a wide variety of beautiful types.
Although caring for them is straightforward and easy compared to a lot of species, we encourage all fishkeepers to understand the needs of their pets so they can live long and happy lives.
This Molly fish guide covers tank setup, food, lifespan, and breeding. Keep reading to find out about this wonderful species.
Recommended Molly Fish Care Items:
Molly Fish Overview
Mollies are some of the most popular freshwater fish for beginners. They’re also a popular option for freshwater community tanks. Molly fish are from the genus Poecilia, in the Poeciliidae family. There are several species within the genus.
In the wild, Mollies are mostly found from Southern United States down into Central America. They thrive mainly in freshwater environments but are capable of temporarily surviving in sea water. They typically inhabit slow-moving tropical waters that have a lot of vegetation that they can use for cover to hide from predators.
Like guppies, mollies are livebearers. This means that instead of laying eggs, they give birth to live young straight into the water. Guppies and mollies also occasionally interbreed under certain circumstances, which has been confirmed genetically.
Most Molly varieties can be found for $2-$4 per fish, though rarer species cost more.
Molly Fish Appearance
When most people envision a molly, they picture the common molly. It is readily available and sold at almost every local fish store. Most molly species have a similar shape and body style. They have a body that is widest at the mid-section, with a triangular-shaped head. Their body tapers to a narrow point at the snout and towards their fan-shaped tail.
All captive mollie species have been interbred over centuries, resulting in many different color varieties and fin shapes. Some of the most common varieties are solid black mollies and sailfin mollies with orange-bordered dorsal fins.
Types of Molly Fish
Through selective breeding, you can purchase many different varieties of mollies. They primarily fall under two main groups, according to their fins:
- Shortfin mollies – These mollies look most similar platy fish in terms of their body shape. However, they have a deeper chest and belly. As their name suggests, their fins are short and not particularly fancy. They tend to stay pretty small, maxing out at about 4 inches. Color patterns include:
- Gold Dust
- Red Sunset
- Sailfin mollies – As the name suggests, these mollies have larger dorsal and tail fins. They tend to be larger, with some maxing out at about 6 inches long. Color varieties include:
This is just the tip of the iceberg on molly varieties. Two other notes on mollies – specifically balloon mollies and imported mollies.
Balloon mollies are a controversial variety of mollies. As their name suggests, they have a big belly that resembles an inflated balloon. Their big bellies are bloated looking due to a genetic mutation that curves their spines. This spinal curvature compresses their internal organs and causes their bellies to bulge because of the pressure.
Many aquarists see this selective breeding as a cruel decision as the fish are being bred in a way that is completely unnatural. Others argue they’ve seen healthy balloon mollies who are thriving. This is where the controversy comes into play.
One important note is that the majority of aquarists report that balloon mollies don’t live as long as other varieties. Their compressed internal organs make them more susceptible to constipation, which can be fatal.
Because of these facts, we advocate not purchasing balloon varieties.
And one final note around imported mollies. A majority of mollies are bred in Asia and shipped all over the world.
The breeders will often raise them in salt or brackish water, which is freely available whereas freshwater costs money. When you’re selling a fish for $2, every little bit of margin counts.
Mollies can be raised in saltwater. However, this causes big problems when they’re then transferred to completely freshwater tanks. Their kidneys shut down over time because they’re used to high salt conditions.
For this reason, we recommend trying to learn about the rearing conditions of the mollies you’re purchasing so you can better replicate their ideal environment.
How long do Molly fish live?
The average lifespan of a molly is three to five years.
The quality of their care and living conditions will impact their lifespan significantly. Additionally, certain varieties tend to be hardier and longer living than others. Balloon mollies, for example, are notoriously short-lived.
How big do Molly fish get?
Adult shortfin mollies typically reach around four inches in length. You don’t have to worry about mollies getting super big, which they can be kept in relatively small aquariums, making them good options for beginner fish or fish in your kid’s aquarium.
Sailfin varieties can get bigger, reaching lengths in the five or six-inch range.
Molly Fish Temperament
Mollies are generally easy-going fish that can easily fit into your freshwater community tank.
Mollies are shoaling fish. Shoaling means that mollies like to be in the company of other mollies, but they don’t necessarily swim in coordinated fashion together like neon tetras. Because of this, plan to keep a group of at least four mollies together at the very least. More is better, if possible!
In your cohort of mollies, you will want females to outnumber the males. Males can harass females, but it should be less problematic if the females outnumber the males. Keeping an all-male group is not advised, as it can bring out some aggressive tendencies.
Molly Fish Care
Molly fish are relatively easy to care for aquarium fish. However, you need to know what you’re doing. Our guide covers their ideal habitat, including tank, lighting, filtration, and more.
Molly Fish Habitat
When planning the ideal molly tank, it is recommended to try to recreate their natural environment. This principle works well when designing tanks and making sure your fish are eating correctly. Recreate their natural environment for best results.
As a native to the warm waters of the Southern United States and Central America, mollies inhabit multiple habitat types ranging from pure freshwater to brackish water. Their ideal conditions include warm and slow-moving water with a slightly alkaline pH.
They are mostly found in the shallow parts of rivers and streams. They typically inhabit waters with a sandy substrate with rocks, organic matter, and plentiful plants.
What is the best tank for a Molly fish?
This is a two-part answer. For shortfin mollies, you can generally keep them in a tank size as small as 10 gallons.
A 10-gallon tank is suitable for up to four shortfin mollies. If you have a larger tank, don’t be shy about using it for your molly crew; they’ll appreciate the extra space. For a larger group of mollies, plan to increase the tank size by at least three gallons per fish. So for example, if you have a group of six mollies, you would want at least 10 gallons for the first four, and then six more gallons for the remaining two. 16 gallons is not a common tank size so you’d ideally move up to the 20-gallon size.
Sailfin mollies get slightly bigger than shortfin mollies and therefore require more space. You don’t want to shortchange them on space as they’ll get more stressed, leading to illness issues. For sailfin mollies, we recommend a tank size of 30 gallons or more. If you can splurge, get a larger group and put them in a 55-gallon tank.
Ideal Molly Water Conditions
Mollies live in different habitats. They live in rivers, but they can also swim in salt water or the open ocean. This means that they’re quite adaptable. Overall, they like warm waters, neutral pH, and hard water.
Try to maintain a temperature between 75 and 80 degrees Fahrenheit. They also like pH between 7.5-8.5 but they can adapt to conditions that are slightly off from this, as long as they’re consistent. You will stress them out if you vary the water conditions too much chasing the perfect range, so try to keep conditions consistent as much as possible.
- Temperature: 75°-80°F
- Ammonia/Nitrite: 0
- Nitrate: <30 ppm
- pH: 7.5-8.5
Maintaining water conditions is important for your aquarium. Use an aquarium testing kit to test your water and stay on top of any changes. This is especially true in smaller tanks.
What to put in your Molly tank
When thinking about how to decorate your molly tank, you will want to recreate their natural habitat as much as possible. Include live plants in your tank as they can help purify the water and provide hiding places. Driftwood is another great option, and it can be used to provide nooks and crannies for your fish to hide in.
What is the Best Type of Substrate for Molly fish?
Substrate is the material that goes on the bottom of your tank. If you choose to use a substrate, there are two main options for molly fish: sand or gravel.
We recommend an aragonite sand substrate. Aragonite contains calcium carbonate (CaCO3). This substrate will slowly leach minerals that are good for the fish over time.
If you want to use gravel, you can add some crushed coral to the gravel which will help leach beneficial minerals in the water.
Salt in my Molly tank?
Overall, we don’t recommend adding a ton of salt to your tank. Molly fish can live in brackish estuaries and saltwater, but this doesn’t necessarily mean you have to maintain the same salinity as ocean water in your tank.
Mollies that have been raised in saltwater may need more salt. Their bodies are used to living with high salinity. We recommend asking your local fish store about the origin of your mollies to better understand if salt would be beneficial.
How Much and What Kind of Lighting do Molly fish need?
Standard aquarium lighting will be enough. These fish do not have special light requirements. You can simply go with a standard aquarium bulb that is included when you set up your tank or anything special you’ve purchased for your plants. Check out our aquarium lighting guide for more ideas.
What Kind of Filtration do Molly fish Need?
Mollies produce high bioloads so you will need a stronger filtration system than smaller fish like ember tetras. If possible, we recommend using a canister filter for your molly tank. This type of filter will be able to keep the water clean, but it won’t necessarily remove ammonia and nitrites from the water.
Make sure that you have a filter that can hold enough beneficial bacteria to keep up with processing waste put off by mollies. Get a filter that can hold a lot of biomedia and always add as much filtration as possible, maybe even adding on an extra filter, like an internal or sponge filter.
Plants are great for making your aquarium more natural-looking. You can add plants of different shapes, sizes, and colors. Mollies like to hide behind tall plants, so consider taller plants when you’re planning your tank.
Mollies love natural environments so driftwood is an excellent option for adding more hiding spaces.
Molly Fish Potential diseases
Important notice: we are not veterinarians at Aquarium Friend so the information below should be used for general awareness only. If you are concerned about the health of your fish, consult a fish health professional immediately.
Mollies are a relatively hardy fish but this doesn’t mean they’re immune to getting sick. It is important to understand potential fish diseases that might afflict your molly so you’re prepared if you notice any issues.
Potential diseases of molly fish include:
- Ichthyopthirius, or “Ich”
- Molly Disease or “Shimmies”
- Bacterial Infections
Molly disease, or the shimmies, is a unique disease to mollies. It’s usually attributed to unstable water conditions like ammonia spikes or unstable temperatures. It is characterized by less swimming, with the fish often wiggling and shimmying in one spot. It is not infectious and typically clears up once the underlying water issue is resolved.
Molly Fish Feeding
In nature, mollies mostly eat plants and algae. Feeding them Spirulina or spinach will help them stay healthy. They also enjoy eating algae in the aquarium although they’re not as effective algae-eaters as plecos and other fish.
Mollies have a voracious appetite and will eat almost anything they can fit in their mouths. A varied diet is best for mollies, offering both plant-based food and some sources of protein. Here are some ideas:
What We Like About This Food
- Provides necessary nutrients for Clown Plecos
- Very affordable
- They sink to the bottom of the tank to appeal to bottom feeders
The most important point to feeding your mollies is variety! One week you might put one type of food in their tank and the next give them something different, like flakes or pellets. This will ensure they get all of the nutrients and vitamins they need.
How Often Should I Feed my Molly fish?
Mollies should be fed at least once a day. They’re voracious eaters but only feed as much food as they can consume in a few minutes. If they easily consume their food in one meal, you can also try feeding them twice a day (but smaller servings).
Be careful not to overfeed your molly fish. Overfeeding is linked to constipation and water quality issues, so try to let your fish finish their food before you feed them again.
Molly fish Tank Mates
Mollies are easy-going fish, which makes them ideal community fish tank members. They get along with most fish and won’t bother other tank mates too much.
Mollies are very social fish and will be happier if kept with other mollies. We recommend one male for every three females. Males bother females by trying to mate with them. Having several females means that not all the males are bothering one lone female. Besides that, mollies are pretty easy to include in community tanks!
Good tank mate options include:
- Cory Catfish
- Most Tetras such as Neon tetras, Ember tetras, and Black skirt tetras
- Bristlenose Pleco
- Celestial Pearl Danio
- Dwarf Gourami
However, this doesn’t mean you can pair mollies with every fish under the sun. These fish are definitely no-go’s as they will likely try to eat your molly:
Molly Fish Breeding
Breeding mollies in your tank is relatively easy once you have a male and female. They’re similar to platies in that aspect. The only real trick to breeding them is getting the right conditions for it. Oftentimes, breeding occurs with no intervention from humans.
Mollies are livebearers. That means that the female will hold the eggs in her belly (instead of depositing them around the aquarium) until the babies are ready to hatch. When the time comes, the female will release free-swimming baby mollies into the water!
How to Sex Molly fish
Mollies are easy to sex. Females have an anal fin that spreads into a fan, whereas a male’s anal fin comes to a point. Females tend to be larger whereas males tend to be smaller. Most females also have a “gravid” spot, which is where they hold their young during pregnancy. The gravid spot is a dark area that is essentially the molly’s womb.
Pregnant females will be even easier to distinguish. They’re larger than the other females with swollen bellies.
The Breeding Process
The first step is finding a male and female molly so they can mate. Males have pointy anal fins while females have fan-shaped anal fins. Once you have a potential breeding pair, we recommend putting them in a breeding tank. The water in the breeding tank should be clean.
Raising the temperature might increase the chances of a successful mating, but don’t go higher than 80°F or you will stress your fish. When the female is ready to mate, she will allow the male to fertilize her eggs. Size matters with mollies and females will often choose the largest male if given the option.
After mating, it is time to play the waiting game. Plan to wait about 35-45 days before the fry are released into the water. You can expect anywhere between 50 – 100 fry.
You will want to separate the fry from the adults or they will get eaten. A breeders box is a great option for pregnant mollies. Once the fry are born, they can swim out of the box through small holes but the adults stay trapped inside, keeping the babies safe.
Molly Fish FAQs
Are Mollies aggressive?
Mollies are very peaceful fish. They prefer to be kept in groups of at least four, and will swim around the top of the tank (or hide) if they feel threatened. They’re a great option for community tanks.
How do I keep my Mollies happy?
Ensure your mollies are kept in a tank with the same conditions as nature intended – a large community tank that has good aeration and filtration. Feed them a varied diet with plenty of plant-based foods. Temperatures should be around 78 degrees Fahrenheit. Also, mollies are shoaling fish so they need a small group (at least four mollies) in order to feel safe.
Do Mollies clean the tank?
Mollies are definitely not stars on the list of algae-eating fish. However, they will eat algae from rocks and plants. They won’t be as effective as your plecos and other famous algae eaters, but they will contribute to algae removal efforts.
Do Mollies sleep on the bottom of the tank?
Yes, mollies will sometimes sleep on the bottom of the tank. Sleeping fish tend to remain upright and balanced, whereas dead or sick fish will lay on their sides. Other reasons for mollies gathering on the bottom of the tank include a temperature drop or water condition problem. They’re otherwise very active fish and tend to be pretty active in the tank.
Do Molly fish need light at night?
Molly fish don’t need light at night. Their preference is natural conditions, which means there would typically be darkness for a portion of the day. By mimicking night, you can encourage your mollies to rest and recharge.
Do Mollies stay at the top of the tank?
Mollies spend most of their time in the middle and upper parts of the water column. If they’re exclusively staying at the top of the tank, it might be a sign they’re uncomfortable with changes in their environment or they might prefer to be at the top of the tank because that’s where all of their food is.
Are Mollies the right fish for me?
In this guide, we’ve discussed the basics of Molly fish care to help you get started with your first pet molly. We hope it helps take some of the guesswork out of owning your own aquarium and ensures that you have a happy healthy tank for years to come!
We discussed molly fish ideal living conditions, including tanks, breeding, feeding, decorations, and more. We hope this was helpful and you feel prepared as a molly owner! Do you have a molly tank? What is your favorite part about owning mollies? Let us know below!