Keeping Tropical Fish: How to Care for Molly Fish

 

Mollies are a popular freshwater fish among beginner and experienced aquarists alike. With their peaceful nature, vibrant colours, and ease of care, it's no wonder they’re often found gliding gracefully through aquariums around the world. In this guide, we’ll take a deep dive into what makes molly fish special, how to care for them, and how to successfully breed them.

 

Molly / Mollies (Poecilia sphenops)

 

What Is A Molly Fish and Where Do They Live?

Mollies, scientifically known as Poecilia sphenops, belong to the Poeciliidae family of live-bearing fish. They are native to Central America, especially in regions from Mexico down to Colombia. In the wild, mollies live in a variety of habitats, including rivers, estuaries, and even brackish (slightly salty) water environments.

Today, due to selective breeding, mollies are widely available in pet stores around the globe and are adapted to typical freshwater home aquariums.

 

What Do Mollies Look Like?

One of the best things about mollies is their variety of colours, shapes, and fin styles. Mollies generally have a slim, elongated body with a rounded belly and a pointed head. They have fan-shaped tails and a moderate to large dorsal fin.

Body Shape by Type:

• Common Molly: Streamlined and torpedo-shaped.

• Sailfin Molly: Longer body with a large, sail-like dorsal fin.

• Balloon Molly: Short, rounded body with a bloated belly.

• Lyretail Molly: Slim body with elongated tail fins.

 

Note on Balloon Mollies

Balloon mollies are popular for their unique, rounded appearance, but their body shape can make them more prone to certain health issues, such as swim bladder problems. Because of their shortened, curved bodies, they are often slower swimmers than their more streamlined counterparts.

This slower movement means they can be outcompeted for food or even bullied by more active or aggressive tank mates, so extra care should be taken when choosing companions. Aim for peaceful, calm fish and ensure food is distributed evenly during feeding.

 

Molly Fish Colours and Varieties

Mollies are known for their diverse range of colours, patterns, and fin shapes, making them a favourite among aquarists who want a vibrant and visually interesting tank. Over the years, selective breeding has produced a wide variety of mollies to suit nearly every aesthetic preference.

While the list below isn’t exhaustive, it highlights some of the most popular and widely available molly types you’re likely to encounter when choosing your fish:

• Black Molly: Completely jet black, sleek, and elegant.

• Dalmatian Molly: White with black speckles, resembling a Dalmatian dog.

• Sailfin Molly: Known for its large, dramatic dorsal fin and often larger body.

• Balloon Molly: A rounded body shape that gives a “balloon-like” appearance. Available in many colours such as Gold, Silver, Marble etc.

• Gold Dust Molly: A striking gold and black combination.

Mollies typically grow to 2.5 to 4.5 inches, depending on the variety. Sailfins tend to be the largest.

Here at Complete Koi & Aquatics, we stock a variety of beautiful molly fish, each with its own unique colours and patterns. If you're having trouble choosing between different types, why not visit us in person? Seeing them up close can help you decide which variety best suits your tank. Our team is always happy to offer advice and answer any questions you might have.

 

What Do Mollies Eat?

Mollies are omnivores, meaning they thrive on a diet that includes both plant and animal-based foods. To keep your mollies healthy, offer them a varied diet.

Feeding them once a day with a high-quality pellet, such as our Tropical Crumb food, provides essential nutrients they need on a day-to-day basis.

Additionally, you can occasionally supplement their meals with protein-rich treats like brine shrimp, daphnia, or bloodworms. However, these should be given sparingly and not as a staple part of their diet.

Providing a varied diet helps brighten their colours, strengthen their immune system, and ensures they receive all the essential nutrients needed to thrive.

 

What Fish Can I Keep with Mollies?

Mollies are peaceful community fish and do well with other friendly species. Good tank mates include:

• Platies

• Guppies

• Swordtails

• Corydoras catfish

• Tetras (like neon tetra or black phantom tetra)

• Gouramis (dwarf varieties work best)

• Snails and peaceful shrimp

Mollies thrive in calm, peaceful environments, where they can swim and interact without stress. To maintain a harmonious tank, it's important to pair them with similarly tempered, non-aggressive fish that won't nip at their fins or compete aggressively for food.

Choosing compatible tank mates not only reduces the risk of injury and stress but also allows mollies to display their natural behaviour and vibrant colours more confidently.

 

What Fish Should I Avoid Keeping With Mollies?

While mollies are generally peaceful, it's important to avoid housing them with certain types of fish that may pose a risk to their well-being:

· Aggressive species, such as cichlids including Convict Cichlids, Rainbow Cichlids, and Parrot Fish. These fish can intimidate or harm mollies.

· Fin-nippers like Tiger Barbs should be avoided, especially if you're keeping long-finned varieties like Sailfin or Lyretail mollies, which can become easy targets.

· Large predatory fish, such as Bichirs, Tiger Shovelnose Catfish, or Oscars, may view mollies as food, either intentionally or by mistake. If a fish can fit a molly in its mouth, it’s best to avoid housing them together.

Although mollies are social and generally easy-going, they can become boisterous during mating, so it's important to choose tank mates that are calm but confident and not easily stressed.

If you’re unsure which tank mates are suitable for your mollies, feel free to visit us in-store or give us a call on 01204 582105, we’re always happy to help. Alternatively, you can click here to view our contact information and get in touch online.

 

How Many Mollies Should Be Kept Together?

Mollies are social and should be kept in groups. A good rule is to keep at least 3–6 mollies, though more is always better if your tank size allows.

When keeping both males and females, aim for a 2:1 female-to-male ratio. This helps prevent the males from constantly chasing and stressing the females. For example, if you have two males, you should keep at least four females.

 

How to Breed Mollies

Breeding mollies is quite straightforward and often occurs naturally in community tanks. As livebearers, mollies give birth to free-swimming young rather than laying eggs. If you have both males and females in your aquarium, chances are they’ll breed without any intervention. While no special steps are needed to trigger breeding, the newborn fry may need extra care and protection to ensure their survival.

 

How to Tell if a Molly Is Pregnant:

• A noticeably rounded belly

• A visible gravid spot (dark area near the back of the abdomen)

• Hiding behaviour or restlessness – sitting in the same spot for a prolonged period of time

• Loss of appetite close to birth

 

How Long Are Mollies Pregnant?

Mollies are typically pregnant for around 4 to 6 weeks (approximately 28 to 40 days). They can become pregnant again very soon after giving birth, so if you have multiple females in your tank, be prepared—you may start seeing baby mollies quite regularly!

 

Supporting Molly Fry (Babies):

• Use a breeding box or move the pregnant molly to a separate tank close to birth

• Once fry are born, remove the mother to prevent her from eating them

• Feed fry fine crushed flakes, baby brine shrimp, or special fry food

If you prefer a more natural approach, both the mother and her fry can be left in the main tank rather than moved to a separate hatchery. Just make sure there are plenty of hiding spots, such as live plants, caves, or decorations, where the fry can seek shelter from adult fish.

While this method makes it more difficult to monitor the fry closely, it closely mimics how mollies breed in the wild. If hiding places are well provided, you might not even notice the fry until they're 3–4 weeks old! By then, they'll be large enough to swim confidently alongside the adult fish.

 

Important Factors for Caring for Mollies

Mollies are relatively hardy fish, but they thrive best when given proper care and a stable environment. As tropical species, they require consistently warm water temperatures, so using a reliable heater is essential. 

A high-quality filtration system is also important to effectively process waste and maintain water quality—especially if the tank is heavily stocked, as this can quickly lead to poor conditions that affect the health of your fish.

Mollies also benefit from a well-oxygenated tank. Adding an air pump or increasing surface agitation through your filter outlet can help maintain healthy oxygen levels, supporting their long-term wellbeing.

Below are the key factors to consider when keeping mollies in your home aquarium:

 

Water Quality:

Regularly test you water with an aquarium test kit to ensure your water parameters are ideal for these fish:

• Ideal pH: 7.0 – 8.5 (Mollies prefer hard, slightly alkaline water)

• Ammonia and Nitrite :0ppm

• Nitrate: Below 40 ppm (ideally under 20 ppm for long-term health)

• KH (Carbonate Hardness): 4 - 8 dKH

• GH (General Hardness): 4 - 8 dGH

Mollies thrive in well-maintained, filtered aquariums where water quality stays stable and consistent. To keep them healthy, it's important to carry out regular water changes of 10–20% each week and routinely test the water for these key parameters.

If you're new to aquarium care or want a refresher, check out our blog “Understanding Your Water Parameters”, where we break down what these parameters mean and how to manage them effectively for the wellbeing of your mollies.

 

Temperature:

• Keep water between 24°C to 28°C

• Use a reliable heater and thermometer to track and maintain these temperatures.

Fluctuating temperatures or prolonged exposure to water that is too cold can be extremely stressful for mollies, weakening their immune systems and making them more vulnerable to illness.

Consistently low temperatures may lead to issues such as sluggish behaviour, loss of appetite, or even serious health problems like fin rot or parasites. To keep your mollies healthy and active, it's essential to maintain a stable, appropriate temperature range within their tropical comfort zone.

 

Tank Size- Plan for Growth and Compatibility:

• Minimum tank size: 40 litres for a small group  

• Larger groups or sailfin mollies need 75+ litres

When it comes to space for mollies, bigger is always better. It's important to remember that the mollies you see in-store are often juveniles, and they still have growing to do. Planning ahead by providing a larger tank ensures your fish will have ample space to swim, grow, and thrive as they reach adulthood.

If you're planning to keep mollies alongside other species, it's equally important to avoid overstocking your aquarium. An overcrowded tank can lead to stress, poor water quality, and health issues for all inhabitants.

Not sure what size tank you need or which fish are suitable for your aquarium? Get in touch with our team at Complete Koi & Aquatics—we’re always happy to help you choose the right setup and stocking levels for a healthy, balanced aquarium.

 

Free Swimming and Hiding Spaces:

Mollies are highly active fish that require plenty of room to swim, explore, and display natural behaviours. To keep them happy and healthy, it’s important to create a well-balanced environment that offers both open swimming areas and plenty of cover.

To achieve this, we recommend including the following in your aquarium setup:

• Provide open areas to swim

• Add plants, rocks, or decorations where fish can rest or hide

• Use floating plants for extra comfort and security

 

Maintenance Tips

Keeping your tank clean and stable requires regular care. Here's what you should be doing on a regular basis:

• Perform weekly water changes of 10%, and always add a dechlorinator when refilling to ensure the water is safe for your fish.

• Clean the substrate using a gravel cleaner or siphon to remove any waste build-up that may be trapped in the gravel.

• Rinse filter media monthly in old tank water to prevent clogging and maintain efficiency. Replace sponges or other media as needed.

• Regularly test your water parameters using a reliable test kit—keep an eye on ammonia, nitrite, nitrate, pH, KH, and GH to maintain a healthy and stable environment.

 

Final Thoughts

Molly fish are vibrant, peaceful, and full of personality, making them a fantastic choice for most freshwater aquariums. It’s easy to see why they’ve become one of the most popular species in the hobby—their wide range of colours and patterns, combined with their gentle temperament, make them a great fit for almost any community tank.

With the right care, a balanced diet, clean, stable water, and plenty of company, your Mollies will thrive and bring life to your aquarium!

 

For more information and advice on aquatics, check out the rest of our blogs…

A Beginner’s Guide to Setting up an Aquarium | Can I Use Tap Water In My Aquarium? | The Best Freshwater Fish For Your Aquarium