Common Names: Mollies
Scientific Name: Poecilia sp
Minimum Tank Size: 20 Gallons
Care Level: Easy
Max Size: 4 Inches
Temperature: 70-82 F
Tank Level: All
Colors: Black, White, Orange
Breeding: Extremely Easy
Molly fish are popular because you can breed them with the dozens of species that exist, creating a range of unique colors, patterns, and fin styles.
Furthermore, Mollies get along well with others of the species and are docile with other fish. So, they’re an excellent option to include in your tropical fish aquarium if you’re seeking maximum variety.
Mollies are also a great species to kickstart your journey as a new tropical fish owner. They’re pretty hardy, so if you accidentally mismanage the tank parameters, your Molly fish will likely survive as long as they don’t have to withstand too much time in suboptimal conditions.
If you spot Mollies in the wild, it’s unlikely you’ll be impressed with their coloration. These fish have silver bodies without much shine, although males turn a bluish-green color during mating season.
They also have black dots around their sides and dorsal fins.
However, thanks to selective breeding, you can encounter Mollies with an array of exciting colors and patterns. Some of their most popular colors include:
While different varieties of Molly fish have varying appearances, as a whole, these fish display the same basic appearance. Their features include:
- Thick abdomens
- Pointed heads
- Upward-turned mouths
- Angular dorsal fins
- Round fins
Over 40 known varieties of Molly exist, with likely many more to come thanks to creative breeding. Below are some of the most common types of Mollies that you’ll encounter at your local pet store.
You guessed it—the black Molly has black scales, although it isn’t uncommon to encounter them with scales of other colors sprinkled across their body. These fish also have a classic round belly and a broad, t-shaped tail.
Balloon Belly Molly
Balloon belly Mollies have an overweight-looking appearance due to their extended bellies. They have a fancier tail because of a lyre-shaped fin, and you can purchase these fish in yellow, white, or black.
You can expect this variety to reach up to 3-inches in length.
Gold Doubloon Molly
The gold doubloon has unique markings, with a primarily deep yellow front half of the body that fades into a black back abdomen and tail. The tip of its wide, triangular-shaped tail has a yellow outline for added beauty.
Creamsicle Sailfin Molly
The creamsicle sailfin Molly is a showstopper, boasting gold scales peppered with white accents. It also has a pitchfork-looking tail, with a long point on the uppermost and lowermost parts.
Molly fish are endemic to the southern part of the United States, spanning down to Mexico’s Yucatan peninsula.
Initially, scientists classified Molies under the genus name Mollinesia. However, they’ve recently changed their genus to Poecilia, which shares the genus with guppies.
A part of what sparked that change is that guppies and Mollies sometimes have babies together, proving how genetically similar they are.
Mollies are primarily freshwater fish, although they can survive in brackish water and spend a short amount of time in saltwater.
It’s common to encounter Mollies in the wild living in bodies of water such as:
In some cases, they even live in ditch water. They’re not particular about their substrate, although they often live in water with muddy or sandy bottoms and cloudy water.
Molly fish are surface dwellers, so they prefer water with loose vegetation. Some of the Molly fish’s favorite aquatic plants include:
- Java fern
Mollies eat plants and algae, so it’s a good idea to include them in your tank so that they can have a hiding place and a snack.
Mollies vary in size according to the variety. If you have a sailfin variety, you can expect your Molly to be longer than average, given that the tail grows so long.
However, as a general rule, you can expect Mollies to grow 3 – 4 inches long, although the range can be as much as 2.5 – 5 inches.
Typical animal dimorphism is flipped on its head within any Molly variety, as the females are always bigger than the males.
So, before bringing home your new Molly fish, make sure to ask whoever you’re buying them from how large they expect them to grow. That way, you’ll know how many Mollies you can comfortably fit in your tank.
Molly fish average a lifespan of 3 – 5 years.
The exact amount of time your Molly will live involves several factors, including the variety of Molly, their nutritional intake, stress levels, and water quality.
Assuming you meet these ideal requirements for your Mollies, they’ll likely live longer in captivity than in the wild.
However, new fish caregivers who don’t take the time to learn about the Molly fish’s requirements may find that their Mollies live at the lower end of the lifespan timeframe.
Molly fish breed quickly and produce live young. So, it’s essential to tell the difference between male and female Mollies if you don’t want dozens of baby fish swimming around your tank.
To determine the sex of Molly fish, you’ll need to observe their anal fin. Perhaps unsurprisingly, this fin sits beside their anus, on the backmost portion of their underbelly.
If you’re looking at a female Molly fish, her anal fin will appear in the same style as the rest of her fins. But in the case of males, they have a narrower anal fin that points back towards their tail.
Male Molly fish are also smaller than females. However, you need to ensure you’re comparing Mollies of the same species and age to use size as a gender determiner.
Overall, Mollies are peaceful fish. However, they may show signs of aggression, including fin nipping if you overcrowd their tank or fighting during feeding time if they don’t have enough food.
The nipping may come in the form of biting other fish within or outside of their species.
Of the two sexes, male Molly fish tend to be more aggressive than females. That’s because they’ll often chase other males of their species to win the attention of a mate.
That said, Molly fish are social by nature and appreciate the company of other fish as long as you give them enough space.
In fact, you should aim to purchase a minimum of four Mollies so they can school together in your tank.
Although Molly fish have a relatively high tolerance for the various tank and water conditions, follow the recommendations below to help them thrive and live as long as possible.
Minimum Tank Size
You should offer a group of four Molly fish at least ten gallons of water. If you want to include Mollies in your tank, four should be the minimum number you purchase.
You should then factor in three more gallons of water for every one Molly you add.
These are the minimum tank requirements—the more space you can give your Mollies, the happier they’ll be.
Furthermore, it’s crucial to factor in room for any other fish or crustacean species in your aquarium.
Once you settle on the best-fit tropical fish tank size, you’ll need to prepare the water for your Mollies by ensuring the following parameters are in order.
Molly fish love warm temperatures, so you should try to keep the water between 75 – 80 degrees Fahrenheit.
Technically, Mollies can survive in water between 70 – 82 degrees.
However, too cold or hot water can stress out your fish, sparking infections, parasites, and disease.
Since it’s unlikely that you keep your house as warm as Mollies need, a heater is the best way to ensure the water stays at the right temperature.
The ideal pH window for Molly fish is 7.5 – 8.5, meaning that they prefer slightly alkaline water.
Nevertheless, different types of Mollies can have varying ideal pH preferences. So, speak with your fish vendor to determine the best-suited pH level for your Mollies.
To ensure the pH is stable, you should regularly check your tropical fish tank’s water every 1 – 2 weeks.
Molly fish can live without salt in their water, just like they can survive with some salt (though they prefer freshwater).
However, since Mollies prefer hard over soft water, you may need to add some cichlid salt to the water to help harden it.
In some cases, you may need to add aquarium salt to your tank to cure your Molly of a disease or infection. In this case, you should use the salt as a temporary addition, cycling out the water once your fish heals.
Now that you’ve picked out your tank and filled it with the perfect water balance that Mollies require, it’s time to spruce it up with the following additions.
Mollies have little preference for substrate, given that they’ll live at the top of your tank. Feel to choose any aquarium safe substrate that fits your fancy and the aesthetic of the tank.
However, these fish appreciate having live plants. So, it’s wise to use sand as a substrate, given that live plants thrive in it.
Alternatively, you can use a gravel-based substrate, preferably from an already established tank. That way, you’ll introduce good nitrifying bacteria into your tank, helping with your fish’s acclimation.
As with substrate, Molly fish typically won’t bat an eye at decorations that you put at the bottom of an aquarium.
So, feel free to add whatever decor you’d like to the bottom of the tank.
The best “decorations” for Mollies are live plants, which we’ll get to next. However, you can also place floating logs or other tall decorations in the tank that they can hide in.
That said, it’s vital to maintain a balance between places for your Mollies to hide and open space at the top of the tank to swim comfortably.
Live plants are like a playground for Mollies, as they enjoy dashing through them. Plants also make an ideal place for Mollies to rest and hide, reducing what could otherwise be a stressful situation.
When choosing live plants for your Molly, we recommend aiming for floating or tall-growing plants. Just prepare to cut back the plants as they grow, for you don’t want to block the water’s surface entirely.
It’s also an excellent idea to grow algae in your tank. That’s because algae are one of the Molly fish’s favorite foods in the wild, so it’s a way to offer them constant access to a healthy treat.
Since Mollies in the wild tend to live in areas with aquatic plants and murky water, they have few lighting requirements.
So, lighting is often more advantageous for the owner than the fish, given it’ll help you see them better.
Should you decide to include a light in your Molly fish’s tank, aim to use one watt of light per gallon of water. That way, you won’t overwhelm them with too much brightness.
Furthermore, you should keep your light on a timer or remember to shut it off every night. Molly fish are diurnal, so you could end up with stressed-out, sleep-deprived fish if you leave their light on all night.
Setting up a filtration system in your tropical fish tank is essential, given that, unlike the outdoors, the water is in a contained space, and waste has no other escape.
A filter is beneficial for the following reasons:
- Removes ammonia and nitrate
- Takes chemicals out of the water
- Whisks away physical debris
- Increases oxygen levels
The three filters you should include in your tank are biological, chemical, and mechanical. These will work together to add good bacteria into the water to eat harmful substances and physically remove other dangerous items.
You should purchase a heater for your Molly fish tank in almost all cases.
Mollies require a higher water temperature than what many people keep their home set at.
You should also purchase a leave-in thermometer for your tank when using a water heater. That way, you can monitor the temperature daily if your heater goes on the fritz.
Molly fish in their natural environment eat an omnivorous diet, including algae, plant particles, and invertebrates.
Although you can offer your Mollies 24/7 access to algae and plants in their tank, you’ll need to supplement the protein side of their diet.
Most people choose fish flakes designed for Molly fish, as they’re easy to store and offer Mollies well-rounded nutrition. However, it’s also beneficial to feed your Molly fish non-processed foods.
Examples of some excellent food choices that you can feed your Mollies include:
- Brine shrimp
Your fish will love you for feeding them these treats, and there’s a good chance the added boost of nutrition will improve their quality of life.
Mollies have small stomachs—usually no larger than their eyeball—requiring little food to feel full. However, like many humans, Mollies will take advantage of the opportunity to overeat if they have tasty enough food.
For this reason, we recommend giving a group of 4 – 6 Mollies one pinch of fish flakes once or twice a day and waiting for two minutes. Then, remove any excess food that they didn’t eat.
You’ll soon learn to eyeball how small of a pinch of food you need to give them to balance keeping your fish full and not overfeeding them.
Breeding Mollies is rewarding, given that females can give birth to more than 100 babies at a time. These little fish, called fry, have live births, a unique experience in the fish world where many come from eggs.
To start the breeding process, set up a special breeding tank. Then, select one male and at least three females.
Having multiple females in the tank will help reduce their stress, given that the male won’t be after them all the time.
You can expect the females to give birth within 3 – 5 weeks after becoming pregnant. Although it likely won’t be possible to time it so that you’re present when she gives birth, gently remove the adult fish and place them back in their regular tank once you see fry in the tank.
That way, you won’t have to worry about the adult Mollies eating the babies, which is a common practice.
To further protect your young fry, make sure the breeding tank has plenty of vegetation, both at the top and bottom of the tank. That way, it offers a place for the baby fish to hide.
If you follow the information we’ve shared here, you’re well on your way to having Molly fish that live longer, stress-free lives. However, diseases can still happen, especially if your tank parameters are out of whack.
So, if you have a sick Molly fish on your hands, below are some of the most common diseases.
Velvet disease results from a parasitic infection, where Oodinium crawls beneath your fish’s scales and gets into their body. The result is a series of yellow circles.
Sadly, the outcome isn’t often positive for velvet disease, given that it can be hard to see the yellow cysts on your Molly fish until the problem gets bigger.
To treat velvet disease, add a copper-based medicine to the water, change up to 90% of the water, turn off your aquarium light, and hope for the best.
Ich, which people also call ick and white spot disease, is another illness caused by a parasite. Like velvet disease, it often occurs when you introduce an infected fish or plant into your aquarium.
Luckily, ich is easier to cure. To identify ich, check for white spots on your Mollies. You’ll likely also notice them rubbing their bodies against hard surfaces.
Curing ich involves applying ich-specific medication to the water. You should also gradually raise the water temperature to 80 degrees Fahrenheit, as ich can’t live at such high temperatures.
Despite its name, a bacteria causes mouth fungus in Mollies, causing white streaks around your fish’s mouth and sometimes their body.
Other signs of mouth fungus include:
- Rapid breathing
- Jagged fins
- Mucus production
You’ll need to treat the water with an antibiotic and add one teaspoon of aquarium salt per gallon of water every three days. Then, if and when your Molly shows signs of improvement, change 50 – 70% of the water.
Tail and Fin Rot
Tail and fin rot is painful for Molly fish owners to look at and even more challenging for your infected fish. A fungus or bacteria can cause it.
Typically, physical injuries or high ammonia concentrations in the water spark this condition.
You can tell that your Molly has tail and fin rot if you notice:
- A disheveled-looking tail
- Parts of the tail sticking together
- Milky secretions from the body
Treating your Molly fish for tail and fin rot involves quarantining them and dousing the water in antibiotics or fungal medication. You should then change 20 – 50% of the water after the treatment.
Sadly, your fish’s fins and tail won’t grow back even if you get the bacteria or fungal infection under control.
Potential Tank Mates
As long as you provide them with a large enough tank, Molly fish are notorious for getting along well with other fish. You’ll want to aim to choose fish species of a similar size to avoid them eating your Mollies.
Some excellent fish to add to your Molly tank include:
- Keyhole Cichlids
Remember, Mollies and guppies share the same genius. So, you can expect them to produce babies if you keep them in the same tank (although the other fish will likely eat them before they get too big).
When introducing new fish to your Molly tank, make sure to stay near your aquarium for the first few hours to tend to any fish conflicts, should they happen.