Are Mollies Aggressive? How Can You Keep Them Calm?

Why is My Molly Attacking Other Fish

Are Mollies Aggressive? How Can You Keep Them Calm?7 mins read

Fact checked by
Sydney Perry
Reading Time: 9 minutes
Are mollies aggressive
Image from Flickr

Mollies are one of the most preferred freshwater fish species, alongside Guppies and Platys when it comes to the aquarium trade. Well known for their shoaling behavior, relatively peaceful nature, and vibrant solid color, Mollies are ideal for beginners and more experienced aquarists alike.

They are ideally easy to keep for beginners and a hardy fish species with a medium to small size making them ideal for community tanks and smaller tanks, and an exquisite addition to enhance the overall aesthetic beauty of your tank.

Not uncommon, many aquarists whether Beginners or more experienced have come across some form of aggression in their Mollies, believing that the Molly species itself is aggressive, which is not entirely true.

There is much to be said about aggression in Mollies, as many studies have shown that certain triggers or conditions can result in aggression in Mollies, especially in communal tanks.

Thus, it is vital to provide Mollies with the correct care and living environment to avoid aggression.

Considering how living conditions and certain behavior patterns in Mollies may affect their aggression levels, it becomes more evident that the first aspect to address is their species and origin, as well as their care requirements before getting into specifics.

Mollies As A Species

There is much to be said about Molly as a species, falling under the scientific name, Poecillia sphenops, and two types, the short-finned or common Molly, and the Sailfin Molly. The Sailfin Molly has a large and prominent dorsal fin, with the common Molly has a shorter and more flowing dorsal fin.

Mollies originate from Columbia and Mexico inhabiting streams and more brackish coastal waters. The original Molly has a duller silver color in comparison to the color variants available in the Aquarium Trade.

Mollies have a similar appearance to their close live live-bearing relatives Platies, Swordtails, and Guppies, and most aquarists prefer them for their strong individual personalities.

Colors And Variations

Mollies are said to have standard sizes of around 2.5 to 3.5 Inches (6.3cm to 8.8 cm) in males, and larger females of around 4 to 5 (10 – 12 cm) inches in females, with regards to different sources of information.

As a result of selective breeding by Aquarists, the Following color variations are mainly found in Mollies

  • Black
  • White
  • Gold
  • Dalmatian

Henceforth, there is the distinct common Molly and the Sailfin Molly with its large dorsal fin resembling a sail.

When digging further into Molly variations there are similarly Mollies with different tail shapes, such as the Lyretail Molly that has more of a long double pointed tail, as opposed to the short-rounded tail of the common molly.

An interesting color is a Midnight or Black molly, a melanistic breed originating from hybrids between variations and the original species.

General Care Tips For Mollies

General care tips for mollies
They need a sizeable tank of between 20 and 30 gallons for a group of four or more with space to move around, as they are active. Image from Flickr

Before getting into Molly’s aggression and how to keep them calm it is crucial to understand their general needs and behavior to address the situation.

  • Temperament and Behavior – Mollies are live-bearing fish with cannibalistic tendencies, thus they are prone to eating their young, and the young of other species. They are Shoaling fish that need to be kept in groups with a ratio of at least one male to three females, otherwise, males may squabble, or females can be harassed by males for mating. Mollies with regards to information from experienced breeders have individualistic personalities, though according to general overview, they can be slightly aggressive, mostly towards their own species, and sometimes towards other fish species. A few good choices in tank mates for Mollies include:
    • Shoaling Fish – Danios, Tetras, Rasboras, White Cloud Minnows, and Guppies. Smaller fast-moving and shoaling fish are generally not harassed by Mollies, though keeping single small fish species will be ill-advised.
    • Similar Sized Fish – Platies and Dwarf Gouramis. Because they are similar in size and nature, to a certain point, they are excellent compatible tank mates for Mollies, and large enough to stand up for themselves.
    • Bottom Feeding Fish – Kuhli Loaches, some Catfish species, Siamese Algae eaters, and Bristlenose Plecos. The benefit of bottom-feeding fish is the fact that they also help to keep your tank clean by feeding on algae and food and waste scraps on the bottom of the tank.

Then again, some snail species similar make ideal tank mares, Nerite snails are one of your best options as they do not over reproduce and are tolerant of brackish water.

A quick suggestion would be to try and avoid slower-moving and smaller fish species, especially those with long flowing tails, which can quickly fall prey to more aggressive Molly’s harassment, as Mollys are quick and active. Other more aggressive species of fish that are larger may similarly, in turn, be aggressive toward Mollies, making them a less desirable choice.

  • Aquarium Requirements – In their natural habitat Mollies prefer either freshwater or coastal brackish water streams, brackish water usually temporarily when breeding. They need a sizeable tank of between 20 and 30 gallons for a group of four or more with space to move around, as they are active. A pH of between 7.5 and 8.5 is preferred, with warmer water conditions of between 70-82 °F (21-28 °C). You can decorate your tank with fine gravel or sandy substrate, enough hiding spaces, and plenty of swimming space. Live or artificial plants are ideal. Live plants such as Amazon swords, java ferns, Hornwort, Guppy Grass, Marimo Moss Balls, and Anacharis are excellent choices. You can introduce both rooted and floating plants.
  • Feeding – Mollies are Omnivorous and eat mostly algae and smaller invertebrates in their natural habitat. You can feed them quality flakes and pellets along with fresh vegetables, and meat-based foods such as shredded Chicken, Boiled Egg Yolk, brine Shrimp, and Blood worms. An interesting fact about Mollies is that they will spit up their food to indicate that they are full, which makes feeding generally easier.
  • Common Health Issues – Overall Mollies are prone to some common freshwater fish conditions, though they are quite hardy. These are:
    • Ich – A parasite causing White spots on the fins and body of the fish.
    • Swim Bladder Conditions – Mostly due to a bacterial or fungal infection that can cause imbalance and issues staying upright, as well as a bloated abdomen.
    • Fin Rot – Caused by Ammonia burns due to dirty water conditions, or physical damage that causes the tail and fins to become infected.

Treating these conditions will typically require the fish to be placed in a separate tank with clean water conditions, and medication for the specific ailment which you can obtain from a vet or local pet store.

  • BreedingBreeding Mollies is relatively easy, and they will breed in a communal tank with males and females, however, they are cannibalistic towards their young, Thus a proper breeding tank for the fry is required. Mollies are Live-bearing, meaning they give Live births to their young. Mollies have an intriguing mating ritual, with the females generally more attracted to boldly colored males with larger fins. Males will constantly chase after females during mating season to attract their attention for mating, and may even fight off other male suitors in their advances.

Aggression In Mollies

Aggression in mollies
Mostly they are known to be peaceful shoaling fish that swim actively and that may show slight aggression towards their own species, especially when stressed or feeling threatened. Image from Flickr

Molly’s aggression is an intricate subject as much of their behavior will depend on their living environment, their individual personality, and other external factors, that will be discussed in detail. Mostly they are known to be peaceful shoaling fish that swim actively and that may show slight aggression towards their own species, especially when stressed or feeling threatened.

Mollies, especially females tend to form a hierarchy referred to as a pecking order, which especially occurs when you add new Mollies to your existing Mollies, that is not advised according to studies, because the newcomer many times will end up on the short end of aggressive behavior from an already established hierarchy.

Are Molly Fish Aggressive?

To answer the question directly though mostly Mollies are described as peaceful shoaling fish. In some cases, it has similarly been noticed that they can be mildly aggressive toward their species. To distinguish an aggressive Molly,  the following behavior has been noted:

  • Their fins will become rigid and spread out, rather than flowing calmly and hanging down.
  • The coloration of the Molly darkens and brightens, becoming more intense with anger.
  • They may twitch their heads in an unnatural way.
  • The eye color darkens much more than usual.
  • They will exhibit more skittish and erratic behavior than their usual active nature.

Sailfin And Shortfin Molly Aggression

According to scientific research, there is definitely a difference in aggression levels between the Common Molly and the Sailfin Molly species. Sailfin Mollies tend to generally be more aggressive than Common Mollies, especially males. Female Mollies will similarly rather choose a male with larger dorsal fins which will leave the Common Molly males out of the equation to mate.

Female To Female Aggression

Interestingly enough, it is not just Male Mollies that may be aggressive. According to studies done, female aggression is likely an issue especially during mating season as they compete for male attention. The issues were especially noted in Sailfin Molly females. Female Mollies like their close relatives Guppies and other species will usually have an eye for the largest most boldly colored male, and they have a weak spot for males with the largest fins.

Similar to aggression in males over a female it might be noted in females regarding a suitable male. As we can see in human nature “the ladies” may be slightly competitive when it comes to who is the most noted among the males.

What Triggers Aggression In Mollies

What triggers aggression in mollies
Mollies are active and require swimming space. In a smaller tank, they can become stressed and aggressive. Image from Flickr

So as mentioned before, Mollies may be aggressive because of specific external triggers, and it will be found that addressing these triggers can ideally reduce, or completely eliminate aggression in Mollies.

  • Incorrect Pecking Order – Crucial why it is recommended to have a ratio of three females per male. More than one male in a community tank may squabble amongst themselves to establish a dominant male and hierarchy. If this cannot be created more aggression may be noted.
  • Lack of Space – Mollies are active and require swimming space. In a smaller tank, they can become stressed and aggressive. Keep in mind the space of your tank when adding plants, additional decorations, or new inhabitants. Mollies quickly become stressed and aggressive when their aquarium is overcrowded.
  • Improper Feeding Methods and Amounts – If the term “hangry” as referred to in humans rings a bell, then this is what happens with Mollies. Hungry Mollies become very aggressive, especially during feeding time. Hungry Mollies may similarly try to fight off other fish species and Mollies if they get used to not getting fed enough. Overfeeding however is similarly not advised as it can dirty your tank water.
  • During Spawning – It is a natural trait for Mollies to be more on edge when spawning. Males have a raised temperature and will relentlessly chase after females. With a lack of females or too many males, they may become aggressive during this time. Males may show aggression towards other males interfering with their chosen females. Similarly, females can become reasonably stressed when constantly chased by males.
  • Males and Females – Males tend to chase the female Mollies around constantly, becoming aggressive, and almost harassing them. Thus why it is always vital to have more females than males, which in turn will give the females a break. Additional hiding spaces can provide your females with a spot out of sight, to relax away from the advances of males.

How Can You Keep Your Mollies Calmer?

So knowing some of the reasons as to why Mollies become aggressive, here are a few methods that works splendidly to keep Mollies calm and Peaceful:

  • Ensure that you have the correct tank size for your Mollies and other fish species. Larger is always better.
  • Include enough hiding spaces such as Caves, Driftwood, Rocks, or Shipwrecks for Mollies and other fish to hide, should they become harassed by the Mollies.
  • Keep your Mollies in a group of talents three females and one male. Sticking to the ratio if you want more Mollies, more females, and a single male is a better choice.
  • Keep Your Mollies properly fed, feeding them twice a day and ensuring that all get enough to eat before removing leftovers.
  • When your male Mollies have established their pecking order more females can be added, though it is best to add your group of Mollies initially all at once rather than introducing a single new Mollies. Try to get your Mollies when they are juvenile, and not matured.

Last Thoughts

why molly fish aggressive
Mollies are active and require ample tank space to swim, like this Sailfin Molly pictured.

Concluding with a few last words, Mollies are reasonably peaceful and active. Aggression in Mollies will mostly occur because of environmental stressors, incorrect male-to-female ratios, or because they are not getting enough to eat as mentioned. Going through the list of reasons, if all these issues are addressed and the Molly still shows aggressive behavior it may on the rare occasion just be an individual personality trait. A few facts to take with regarding Mollies:

  • Mollies are active and require ample tank space to swim.
  • Proper Hiding spaces are required for other fish species and Mollies that may feel stressed or threatened.
  • The correct male-to-female ratio in Mollies is crucial, one male for every three females, you can always have more females, though be careful with too many males.
  • It is recommended by professionals to rather add all your Mollies to their new home at once, as juveniles, rather than introducing new Mollies as singles.
  • Always keep Mollies well fed, though do not overfeed as this can cause ammonia spikes in your tank that can result in health conditions.
  • Sailfin Molly species are slightly more aggressive than common Mollies generally speaking, with some research backing.
  • Keep tank water clean as Mollies will become stressed, ill, or aggressive in poor water conditions.

Frequently Asked Questions

Are Black Molly Fish Aggressive?
The Black Molly like the common Molly may show aggression the same way and for the same reasons as most Mollies. Though more closely related to the Sailfin Molly, the males may be slightly more aggressive.
Why is My Molly Attacking Other Fish?
Ideally Mollies usually only show aggression towards their own species, but on rare occasions, they may be aggressive towards other fish species. They may bully and attack other fish species because they are stressed, usually resulting from poor water conditions, overcrowded tanks, or inadequate hiding spaces.
Why is my Female Molly Chasing the Male?
If noticing that the roles are reversed and females are chasing after males, which is quite rare, it will usually be because they want to mate, or there is a lack of food.
View sources