Mollies are a fish I’ve kept on and off for over two decades, and they’re far more interesting than most beginners realize. The salt and hard water preference is real โ I’ve seen molly tanks crash when kept in soft, acidic water that would be perfect for tetras. Get the water chemistry right and they thrive, breed readily, and add great color and movement to any community setup.
Mollies are one of the most underappreciated livebearers โ they get lumped in with guppies and platies as beginner fish, which is fair, but there’s something that sets them apart: they’re one of the few freshwater fish that can genuinely tolerate brackish water and even light saltwater. I’ve seen mollies kept successfully in reef sumps. They also do noticeably better with a little aquarium salt and harder water than most fishkeepers bother with. Skip the salt and keep them in soft, acidic water and you’ll eventually see the shimmy disease that mollies are famous for โ a wobbling, shimmying behavior that signals stress or illness. Get the conditions right and they’re virtually bulletproof. Here’s the full care guide.
Brief Overview Of The Molly Fish
| Scientific Name | Poecilia sphenops, P. latipinna, P. velifera, etc. |
| Common Names | Molly, molly fish, sailfin molly, shortfin molly, balloon molly, etc. |
| Family | Poeciliidae |
| Origin | United States of America, Mexico, Colombia, Venezuela, etc. |
| Diet | Omnivorous |
| Care Level | Moderate |
| Activity | High |
| Lifespan | 2 – 5 years |
| Temperament | Peaceful/ semi-aggressive |
| Tank Level | Midwater |
| Minimum Tank Size | 15 gallons |
| Temperature Range | 72 ยฐ to 82 ยฐF |
| Water Hardness | 15 – 30dH |
| pH Range | 7.0 to 8.5 |
| Filtration/Water Flow | Low to Moderate |
| Water Type | Freshwater |
| Breeding | Live bearer |
| Difficulty to Breed | Easy |
| Compatibility | Community tanks |
| OK, for Planted Tanks? | Mostly, may eat soft plants |
Molly Fish Origins and Habitat
The popular molly fish of the aquarium hobby are native to the North and South American countries of the United States, Mexico, Colombia, and Venezuela. They have also been introduced to some Caribbean islands and East Asian countries.
In nature, the molly fish lives in a huge variety of different habitats. Mollies are euryhaline, which means they can live in varying salinity.
They are most often found in freshwater environments, temporarily moving into brackish water in coastal areas. They are also occasionally found living and breeding in pure saltwater too, which is really amazing!
What Do They Look Like?
Molly fish are so variable in shape, color, and markings, that it’s tough to describe just what they look like! I’ll go into a little more detail on the various species and breeds in the next section, but generally speaking, mollies are solidly built mid-sized aquarium fish.
One very noticeable feature is the unusually thick base (peduncle) to their tails. On the opposite end, their heads are sharply pointed in profile when viewed from the side. The head is wide across when viewed from above, with their mouths positioned right in front of their faces.
They also have very big eyes, and their lower jaw sticks out a little further than the top. These are the features that are most obvious in all mollies.
Species
There are about 40 species in the Poecilia genus, including other well-known livebearers such as the common guppy. A handful of common molly species are popular in the aquarium trade, and they have been bred with each other to produce hybrids and various color morphs.
Let’s take a look at some of the more common molly fish types:
Sailfin Molly – Poecilia latipinna
Sailfin mollies have huge dorsal fins that they can lift up to look just like a sail. This is a natural body feature that the males use to impress the females. These beautiful fish are scientifically known as Poecilia latipinna.
Sailfin mollies are relatively small, reaching a maximum length of about 5 inches. This species actually occurs naturally in the United States, living wild as far north as North Carolina.
Shortfin Molly/ Black – Poecilia sphenops
The shortfin or black molly is one of the most common types of molly. Black mollies are smaller than the sailfin, reaching a maximum size of less than 5 inches.
In the wild, they are naturally silvery with some color on their fins. The black molly fish is the most popular color breed of this species.
Giant Sailfin- Poecilia velifera
The giant sailfin molly fish is the largest of the common species. These Mexican fish can reach a length of 7 inches in the aquarium. They have a huge dorsal fin like the regular sailfin but can be told apart by having more fin rays (18-19), and round spots on this fin.
Varieties
The various molly species are able to interbreed, and aquarists have crossed them to create an amazing array of different varieties. Selective breeding has refined the results, creating fish with different body shapes, finnage, colors, and patterns.
Here are a few of the most popular molly types:
- White Molly Fish
White mollies are a pure, bright silvery color. They should not be confused with albinos which have more yellowish color and pink eyes.
- Golden Molly Fish
Mollies are also available in amazing golden-orange colors. They can be uniformly golden colored or flecked with black like the gold dust molly. Specimens that are partially golden and partially white are known as creamsicle mollies.
- Dalmatian Molly Fish
Dalmation mollies are whitish fish that are covered in fine black spots, just like dalmatian dogs. The blotching is quite variable, and these fish are sometimes known as marbled, or salt-and-pepper mollies.
- Lyretail Molly
Lyretail molly fish have elongated rays at the top and bottom of their tail fins. Lyretail mollies are available in all the different colors varieties, which makes them great for aquarists who want that extra bit of flair in their aquarium.
- Balloon Molly
Balloon molly fish come in a variety of colors, including black, white, golden, and marbled. What sets balloon mollies apart is their rounded bodies, almost like an inflated balloon! This is not a natural feature, however, but rather a trait that has been selectively bred for.
Size
Molly fish vary in size depending on their species, variety, and gender. Adult mollies vary from a little over 3 inches, right up to about 7 inches in length. Females are usually larger than males, but this is not always the case.
How Long Do They Live?
The typical molly fish lifespan is from two to five years. This depends on a variety of factors, though, including their level of care, diet, and genetics.
Temperament and Activity Level
Mollies are active and confident fish that are always busy and lively. They are generally peaceful community fish, but here are a few scenarios in which mollies might be semi-aggressive. We have a video from our YouTube Channel that goes into detail about their care and temperature. Check it out below.
Mollies are usually very peaceful with the other species of fish in the tank, especially if they are kept in a group. Males can be a little aggressive with one another when competing to breed, however. They will also harass the females pretty relentlessly, which is normal behavior for the species.
If you don’t mind your fish breeding, the ideal stocking rate is a ratio of one male to two or three females. This will take some of the pressure off the females.
They can also be quite competitive with each other around mealtimes. As long as each fish is getting enough to eat, this isn’t too much of a concern though.
What Are Good Tank Mates
Mollies are great fish for a carefully planned hard water community tank. They are highly versatile and these fish thrive in many different setups provided they are happy with the water parameters.
Choosing the right tank mates for your molly fish is very important too, of course, so read on to learn about some great options.
Best Tank Mates For Companions
The first thing to note is that all tankmates should be comfortable in the same hard water conditions that your mollies prefer. Mollies can be kept in anything from fresh to saltwater, so I’ve grouped these tank mates according to the type of water they live in.
Mollies are euryhaline fish, but most other species are not, so never mix other freshwater fish with saltwater tank mates or vice versa.
Freshwater tankmates
- Other molly species
- Loaches
- Honey gouramis
- Other livebearers like guppies, platies, and swordtails
- Rainbowfish
- Some barbs and tetras
- Other small, peaceful freshwater fish that are adaptable to hard water
Brackish Water Tankmates
- Other mollies
- Chromides
- Gobies
Saltwater Tank Mates
Molly fish are one of the few fish in our hobby than can be converted over to a saltwater aquarium. There are steps on how to do this (more on this later). They make great algae eaters, but they are small in comparison to many saltwater fish and may be bullied. Here are a few possible candidates.
- Other mollies
- Percula clownfish
- Peaceful blennies and gobies
- Royal grammas
Least Compatible Fish For Companions
As adaptable and versatile as mollies are, some fish will not get along with them. Take note of the following tips to avoid any problems:
- Avoid tank mates that need soft acidic water conditions
- Avoid any aggressive tank mates that could eat or bully your mollies
- Keeping mollies with shrimp is not recommended
- Avoid very small and shy fish that are easily intimidated
What Do They Eat?
Mollies are omnivorous fish that eat both plant and animal matter. Sailfin mollies in particular need plenty of plant material in their diet. They are not fussy eaters and actually do a great job of cleaning up scraps and uneaten food in the aquarium.
Let’s take a look at some of the best food sources for these fish.
Prepared Foods
- Fish Flakes
- Wafers
- Pellets
- Granules
Live & Frozen Foods
- Brine shrimp
- Blood worms
- Daphnia
- Blackworms
Frozen bloods are a great source of protein and a fish source fish naturally respond to. Very filling and works for just about any fish
Vegetables
- Zucchini
- Lettuce
- Spinach
Algae
Mollies love to eat biofilm, and surface scum. They are also known to eat other types of algae like green algae, black beard algae (BBA), hair algae, brown algae, and blue-green algae. This makes them valuable members of your clean-up crew!
How Much and How Often to Feed Them
One of the most common mistakes in the fishkeeping hobby is overfeeding. But how do you know how much food your fish need?
Mollies don’t waste time when there’s food around, so if they haven’t finished everything after 2 minutes or so, you can assume you’ve given them too much food.
Sure, they might finish the leftovers later, but what about the food that gets sucked into the filter, settles in the gravel, and otherwise goes to waste? Well, this leftover food tends to rot and results in poor water quality.
That’s why it is best to feed small amounts 2 or 3 times a day, rather than a large amount just once a day.
Setting Up Your Tank
Setting up a great aquarium for mollies is easy because these fish are very adaptable. Nevertheless, there are some important things to know before putting a tank together. Read on for more details.
Tank Size
Molly fish can be kept in tanks as small as 15 gallons, and sometimes even less. In such a small tank, you could keep up to four mollies if you have good water filtration and perform regular aquarium maintenance. A 20 gallon long is a good candidate to start with.
A classic 20 gallon aquarium in its 30 inch long variant. A very popular aquarium.
A larger tank would be a better bet, however, especially if you plan on keeping a bigger school or other species of community fish.
Plants
Mollies thrive in planted aquariums. Beware though, mollies are omnivorous fish and they have been known to feed on delicate, soft-leaved plants. Tougher species like these plans below will do well with Molly fish:
A mix of tall plants like Vallisneria in the background with Java ferns in the midground and some anubias nana petite in the foreground could make a tough and simple but beautiful planted aquascape for these fish.
Substrate
Substrate type is not critical when it comes to keeping mollies. Any aquarium-safe sand or gravel will work just fine. If your water is not naturally hard and alkaline, it is a good idea to incorporate some crushed coral into the substrate.
Decor
Molly fish are not particular about the decorations in their tanks so you can use your creativity to put together an aquarium that suits your eye.
Adding some driftwood and rocks is a great way to create a more natural tank environment, but there’s nothing wrong with putting in some aquarium-safe ornaments and decorations too.
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Water Quality
Mollies are known as hardy fish. So much so that some hobbyists use them to cycle new fresh and saltwater aquariums.
Nevertheless, maintaining high water quality is very important if you care about keeping your fish in good health for the long run. Maintaining great water quality for your mollies relies primarily on good filtration and aquarium maintenance.
Read on to learn more about these, and other important water quality factors.
Filtration
A good filtration system usually means the difference between a sparking, pristine aquarium, and a toxic and dangerous environment for your fish. Aquarium filters work by literally filtering out particles in the water, a process known as mechanical filtration.
You might not know how much more is happening behind the scenes though!
Microscopic life forms known as beneficial bacteria take up position and colonize the sponge and other media in the aquarium filter. These beneficial bacteria make their living by feeding on some pretty toxic chemical compounds (like ammonia) that enter the water through fish waste and uneaten food. This process is known as biological filtration and is very important for your fish.
Air-powered sponge filters, internal power filters, hang-on back filters, and external canister filters can all be used to perform these vital functions. Just make sure the filter you choose is rated to filter your tank size or larger.
Water Parameters
Mollies are adaptable to a fairly wide range of water temperatures from about 70ยฐF to a little over 80ยฐF. This means they can be kept in unheated aquariums in many cases.
I would recommend using a heater to keep the water temperature stable in the middle of that range because they are comfortable in tropical climates with warmer water.
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Mollies can live in fresh, brackish, or even saltwater. In a freshwater aquarium, they must be kept in hard or at least fairly hard water for their long-term health and survival. The alkalinity should be basic, i.e. above 7.2.
Even though they have a reputation as hardy fish, mollies are not immune to the effects of ammonia and nitrite in the water. Aim for the following levels:
- Nitrate (NO3): Less than 20 parts per million
- Nitrite (NO2): Zero parts per million
- Ammonia (NH3): Zero parts per million
Salt
One of the biggest debates around molly fish care is their salt requirements. Many fish keepers will tell you that you need to add salt to the aquarium to keep these fish happy, but this is not necessarily true.
The salt these fish require does not necessarily need to be regular aquarium salt (NaCl), even though they are highly tolerant of various salinities. Calcium and magnesium, i.e. the elements responsible for hardening water are essential, however.
Mixing crushed coral into your substrate or using a product like Seachem Equilibrium is a great way to increase water hardness if your tap water isn’t quite hard enough for mollies.
Acclimating To Saltwater
You may have heard that some aquarists keep mollies in saltwater. As strange as this sounds, it’s absolutely true! Properly acclimated mollies make awesome saltwater fish because they are so affordable and do a great job of eating certain kinds of algae. The video above from Everyday Aquarist goes through the process. I’ll explain more below.
You shouldn’t pick up a couple of mollies from the pet store and drop them straight into a reef tank. Mollies are highly adaptable, but if the change in salinity is too rapid, they could go into osmotic shock.
The most successful techniques involve slowly replacing the freshwater with salt water over a period of one to two days. This gentle acclimation will be a lot less stressful for the fish. Just be sure to provide the fish with an airstone during this process if they are in a bucket or small tank for acclimation.
Slow acclimation can be achieved by siphoning saltwater in from a tank or bucket and allowing the excess to overflow. The saltwater needs to be introduced very slowly, so a drip rather than a flow is recommended. An accudrip acclimator can be used to make the setup easier.
Most of us know that fish and shrimp are sensitive creatures, who don’t do well with sudden changes. The Accudrip Acclimator is here to help adjust your aquatic creatures to new tanks and conditions
Use a refractometer to measure and monitor the salinity of the water. Specific gravity should not go above 1.025.
Aquarium Maintenance
Regular aquarium maintenance is vitally important for keeping healthy molly fish. A weekly partial water change is recommended to keep nitrate levels down, and this is a good time to give the tank a general clean as well.
Remember to treat your tap water with a water conditioner before adding it to the tank. If your aquarium filter media needs to be rinsed out, use tank water that will not harm the hard-working colonies of beneficial bacteria.
Testing
Testing your water parameters regularly is absolutely vital if you wish to keep healthy mollies in the long run. Pick up a master test kit that can be used to measure the following parameters:
- pH
- Hardness
- Ammonia
- Nitrate
- Nitrite
These are the most important parameters to keep an eye on if you are keeping mollies as freshwater fish, but if you are keeping them in brackish or saltwater, you’ll need a hydrometer to measure specific gravity too.
Breeding
It is very easy to breed mollies. These fish are livebearers, which means the females give birth to live fry instead of laying eggs. If you have adult male and female mollies in your tank, and they are happy in their environment, they will breed freely.
Sexing Mollies
Successfully breeding molly fish requires having both males and females. But how do you tell the difference?
Fortunately, sexing these fish is easy once they are old enough to show differences in gender. This usually takes at least a few months with mollies.
Let’s take a look at some of the most noticeable gender differences:
- Male mollies are usually smaller and more colorful than females
- Male mollies have a larger dorsal fin than females
- Female mollies have a well-developed anal fin, whereas the anal fin of males is modified into a narrow, elongated structure known as a gonopodium
- Male mollies will spend a lot of time trying to entice the females. They will chase and swim around them
- A pregnant female molly will develop a large, rounded belly with a characteristic gravid spot just in front of the anal fin. This spot will be very difficult to see on a black molly fish
Raising & Protecting Fry
Molly fry are very small and vulnerable to being eaten by the other fish (including mollies) in the tank. Ideally, the pregnant female should be moved to a separate breeding tank before giving birth.
Alternatively, you can use a breeding box in your aquarium to keep the baby fish safe until they are too big to be swallowed. The fry can be fed a diet of baby brine shrimp or flake food that is crushed up into a powder form.
Preventing Breeding
Female molly fish are pregnant for about 2 months, and can give birth to over 50 fry! This means the population of fish in your tank can rise pretty dramatically if you allow it.
The best way to prevent breeding is to keep only female fish. Pregnant females can store sperm for a few broods after mating, so separating adult mollies will not stop the female from producing fry right away.
Health And Disease
Naturally, you want your molly fish to live a long healthy life, so what are the problems to look out for? Read on to learn more about the health and wellness of these popular aquarium fish.
Evaluating Their Health
Evaluating your fish’s health requires careful observation. Start by looking at the fish’s physical appearance. Try to answer the following questions:
- Do the eyes look swollen or unhealthy?
- Are the fins torn or clamped against the body?
- Does the fish have very long stringy white feces?
- Is the fish covered in white spots or any tiny parasites?
- Is the fish very thin, or heavily swollen (skip this for balloon molly fish!)
If the fish looks physically healthy, there are still behavioral warning signs to look out for.
- Is the fish breathing rapidly?
- Is the fish having trouble swimming or just shimmying at the top of the tank?
- Is the fish flashing and scratching its body against the substrate?
- Is the fish floating or sinking?
- Is the fish avoiding food?
If you answer yes to any of these questions, there is most likely a problem with the fish. You can go through these questions before picking out your fish from the pet store, or just for monitoring your fish at home.
Common Health Issues
Mollies are most likely to suffer health problems when their tank conditions are not suitable. This can be the result of soft water, poor water quality, or a lack of space in a crowded tank.
Mollies are typically bred in brackish waters, and the shock of being put into freshwater at the pet store or in your home is another potential problem. These are all causes of stress for your fish, and stress puts them at high risk of common freshwater diseases and disorders like:
- Ich
- Columnaris
- Velvet
- Popeye
- Camallanus worms
- Swim bladder disorders
- Shimmies/Livebearer/ Molly disease
Where To Buy
Mollies are common and easy to find down at your local pet store. They are affordable fish, although you can expect the price to vary depending on breed and color pattern.
They are also available online of course, which is great for aquarists who don’t have a good local fish store nearby.
FAQS
Are they easy to care for?
Molly fish are moderately easy to care for which makes them a good choice for beginner aquarists who are willing to do some planning. They will thrive if you can provide them with the water parameters and quality they need.
How many mollies should be kept together?
Mollies are social fish that should be kept in groups of at least four fish. You should always keep more female mollies than males, so a group of three females and one male would be the lowest recommended number.
Do mollies need a heater?
Mollies are typically thought of as tropical fish, but if you look at their natural distribution, you’ll see that this isn’t strictly true.
They can be kept in an unheated aquarium if the water in your tank stays consistently within their preferred range of 70-82ยฐF. Using a heater will protect the fish against any cold snaps, and keep the conditions more stable, however.
Can they live in tap water?
Molly fish can be kept in tap water that is naturally hard and alkaline. You should always treat tap water with a water conditioner to neutralize chemicals like chlorine and chloramine, however.
Can mollies live with Betta?
It is possible to keep mollies and bettas together, but they are not ideal tank mates. Even though mollies are usually peaceful fish, they do occasionally nip at fish with large flowing fins.
Final Thoughts
Mollies are hands-down one of the best fish in the hobby. They look great, they’re adaptable, and they’re great for eating algae too! I hope this article has cleared up some of the confusion for you and helps you keep your molly fish happy and healthy.
Do you keep mollies? Tell us about your experiences with these amazing fish in the comments below!
๐ Want to learn more? This article is part of our complete Freshwater Fish Guide โ your ultimate resource for freshwater species, care tips, tank setup, and more.
