Molly Fish Bullying & Attacking Other Fish: 7 Quick Solutions


Aggression among fish is a known issue. For example, I frequently noticed that my molly fish attack and bully other fish in my aquarium. Over the years, I learned why this issue occurs and how to deal with it. Now, I am willing to share my experience.

Molly fish tend to bully and attack other fish when they are stressed. That usually happens when the water parameters are wrong, including the pH, temperature, and ammonia levels. However, mollies will also behave aggressively when they are breeding or sharing a tank with unsuitable tankmates.

As we move forward, I will share seven crucial steps you should take to stop your molly from bullying other fish. That includes measuring the water parameters with the API Aquarium Test Kit (link to Amazon) and adjusting them to molly fish.

Why Is My Molly Attacking Other Fish?

Molly fish are peaceful creatures. While they are somewhat challenging to rear, they have a reputation for living in harmony with their neighbors. If your mollies are bullying and attacking other fish, you have to ask why?

Only by investigating your tank and its inhabitants can you identify the factors that might have driven your mollies to adopt such a violent attitude. Some of the factors that you are bound to discover include:

1. Your Mollies Are Merely Having Fun

What does bullying look like in an aquarium? Are your mollies chasing other fish and possibly even nipping at their fins? Are you so confident that these are signs of violence? Fish are playful creatures, and they will chase each other around without causing any lasting harm to one another.

Don’t be so quick to panic unless you have found clear and irrefutable evidence of aggression and violence in your tank. If your molly is indeed attacking and bullying its tankmates, its victims will spend a lot of time cowering.

The victims won’t eat as much either because they are too afraid to confront the mollies during mealtimes. In the worst-case scenarios, these bullied fish will manifest bruises and marks, with the most prominent being nipped and frayed fins.[1]

If the fish your mollies are attacking are healthy, happy, active, and free of injuries, you can comfortably conclude that your mollies are just playing. In this case, you don’t need to take any action. The behavior is perfectly normal.

2. The Wrong Tankmates

If you know for certain that your molly’s actions are a sign of violence rather than playfulness, consider the tankmates. You have to keep two factors in mind:

  • Size – Fish have a reputation for attacking smaller creatures in the aquarium.[2] This is why aquarists are warned against keeping fish whose sizes vary drastically in the same tank. The larger fish is more likely to confuse the smaller fish for food. 

If your molly’s tankmates are so much smaller, it might be tempted to harass them because they cannot fight back, especially if the molly is stressed. For example, mollies are likely to attack male guppies, which are relatively smaller.

  • Aggressive Nature – As was mentioned above, mollies are known as peaceful creatures. But this is only true if their tank is filled with equally peaceful fish. Aggressive and hyperactive tankmates will encourage a peaceful molly to manifest violent tendencies. 

Some fish attack their neighbors simply because they are bullies. But others become violent as a response to the hostility their neighbors have thrown their way. If a molly’s tankmates are bullies, it will respond with the same behavior, especially if the tankmates are within the same range as the molly where the size is concerned.

3. Your Mollies Are Breeding

Mating is one of the most common causes of aggressive behavior in mollies. Male mollies have been known to harass female mollies when they want to mate. If you have enough female mollies, this behavior isn’t a problem. What might look like hostility is perfectly normal for male and female mollies that are mating. 

Male mollies only become an issue when they don’t have enough females in the tank. They may harass the few females in their tank to death. The male mollies are also more likely to fight one another for the few females in their aquatic environment.

If your mollies are fighting amongst themselves, take a moment to determine whether the victims are female. Males are not only smaller but more colorful. They also have larger caudal fins and a gonopodium.[3] 

If the aggressors are male and the victims are female, you can conclude that the aggressive behavior you keep witnessing is a mating sign. This behavior usually resolves once the female gets pregnant and the mating process ends.

4. There Are Too Many Male Mollies

Of the two, male mollies are more violent. They can become a problem if you force them to live in small groups in a tank with no females. They will fight one another for territory and as a way of creating a pecking order. If you have one male molly, it will attack fish from other species.

5. Inadequate Water Conditions

Poor water conditions can compel even the most peaceful species to attack their neighbors. Mollies are firm fish that can live in fresh and saltwater conditions. They can also tolerate poor conditions such as low levels of oxygen and high hydrogen sulfide concentrations.

However, such poor conditions will induce stress in the mollies if they persist. The creatures are susceptible to nitrite. If you force them to survive in a poorly maintained tank, they will become a menace to all the other aquarium fish.

6. There Isn’t Enough Food

Mollies are omnivores. Like most fish, they have to be fed once or twice a day. If you force them to starve, they will attack other fish in their search for alternate sources of food. If the mollies don’t want to eat their neighbors, the starvation stress will push them to bully and harass other fish.

7. The Tank Is Too Small

If your mollies live in a small, overcrowded tank, they are going to attack their tankmates. This is a typical response among fish. They do not appreciate cramped conditions. Overcrowding forces them to fight for territory, which you don’t want. It won’t take long for a little jostling for territories to evolve into a vicious encounter that may claim the lives of one or more fish.

How Do I Keep Mollies From Bullying Other Fish?

If your mollies keep attacking their neighbors, there are ways to keep them in line, for instance:

1. Adjust The Aquarium Conditions

The best way to keep fish calm is by improving the quality of their water. It isn’t a surprise that your molly is acting out if it is forced to inhabit conditions that it does not enjoy. I suggest keeping the following in mind:

  • Parameters – Mollies require temperatures of 72 to 78 degrees F, a hardness of 20 to 30KH, and a PH of 6.7 to 8.5.[4] If you keep them in hard, alkaline water, they will behave.

To ensure that your water parameters are correct, I highly suggest getting the API Aquarium Test Kit (link to Amazon). That bundle accurately measures your pH, nitrates, nitrites, and ammonia. Within five minutes, you’ll know if something is out of range.

As was mentioned, water hardness is another crucial factor. That is especially true if you are using tap water for your aquarium. To measure that, I use these Water Hardness Test Strips (link to Amazon). They are handy and impressively affordable.

  • Size – Technically, mollies require a minimum of 10 gallons. But the actual size will depend on the type of molly. For instance, while short-finned mollies can live in 10 gallons, sailfin mollies require a minimum of 29 gallons.[5] 

The larger the mollies, the bigger the tank. This is also true for the number of fish. The more fish your tank has, the more space your mollies will require. If you feel that your current tank is too crowded, feel free to check my recommendations for aquarium kits.

  • Salt – Some aquarists have argued that mollies require salt in their freshwater tanks. Apparently, the presence of salt protects the fish from infections, especially mollies that are forced to live in soft, acidic water.[6] Other aquarists have disputed this claim. 

They believe that this myth has been fed by the presence of hybrid mollies in certain circles that are more sickly than regular mollies because of the weakness that was bred into their generic line. Ultimately, if you don’t have a stance on the issue, you can experiment with salt to determine whether or not it benefits your mollies.

2. Get The Right Tankmates

Please keep your mollies with suitable tankmates, peaceful fish with similar water chemistry requirements, and whose size falls within the same range as the molly. That includes guppies, platys, tetras, and swordtails, to mention but a few.[7] Avoid large and aggressive fish like convict cichlids.

3. Choose The Right Numbers

Mollies are social creatures, and for that reason, you shouldn’t raise them on their own. Generally, I suggest keeping the following in mind:

  • Two Genders – If you have a combination of male and female mollies, the male mollies shouldn’t outnumber the females. You need three female mollies for every male molly. This will prevent the male mollies from ganging up on one or two female mollies.
  • One Gender – If you don’t want to breed mollies, you better keep only female mollies in your tank. They are less likely to fight one another.
  • Male Mollies – If you have only male mollies in the tank, keep them in groups of five or more. This will dilute their aggression.

4. Perform Routine Maintenance

I highly suggest that you keep your tank clean. Install strong hang-on-back filters that you can trust to keep pollutants out of the tank. Try to perform regular water changes. Do not permit toxins like ammonia and nitrites to accumulate. 

I also recommend keeping conditioners on hand that can neutralize these toxins whenever they run amok. That is where I usually mention the Seachem Prime (link to Amazon). That fantastic product will take care of chlorine and chloramine and efficiently detoxify ammonia, nitrates, and nitrites.

Another important step would be vacuuming the substrate. Remove waste, leftovers, dead fish, and any other organic matter that might increase the ammonia concentration by rotting. That is a crucial factor since high ammonia concentrations will quickly burn your fish’s gills.

Pay attention to your water source during water changes. Some water sources have chlorine, chloramine, lead, copper, and various other contaminants. If you don’t know the makeup of your water, take a sample to a lab. The test results will provide you with a clearer idea of the conditioners you can use to purify the water before adding it to the tank.

5. Feed Your Mollies Right

Give your mollies food in sufficient quantities. They need a combination of animal and plant matter. Though, they tend to favor plant-based food. You can give them everything from flakes and pellets to bloodworms, peas, zucchini, and daphnia, to mention but a few.[8] 

You can use an automatic feeder if you don’t have the time to feed them yourself. I personally use the Eheim Automatic Feeding Unit (link to Amazon). That excellent device allows me to forget about the feeding obligation and ensures that my fish never starve.

6. Consider Isolation

If you have identified the aggressive molly fish, try pulling it away from its victims with a fishing net. Some fish will stop their assault once you intervene. If this doesn’t work, place the bully in a breeder tank. Keep it in the corner of the aquarium for a few days or weeks. 

In this situation, the bully can see its tankmates, but it can’t attack them. The molly is more likely to behave once you release it. If that doesn’t work, place the violent molly in a separate tank. Some mollies have naturally aggressive personalities, and you cannot fix them.

7. Introduce Some Hiding Places

Give your molly’s victims places to hide. That includes caves, driftwood, plants, rocks, and the like. The presence of hiding places will also help your mollies because it gives them peace of mind, allowing them to overcome stress.

If your aquarium has plenty of hiding places, but you have noticed territorial behavior in your mollies, you don’t need to add even more hiding places to the tank. Instead, rearrange the objects you already have in the aquarium. Turn the lights off and change your fish’s aquatic environment. This will create a level playing field by destroying all existing territories.

Are Black Mollies Aggressive?

Black mollies are not known for being aggressive. However, they may act violently if they live in a tank with several males and very few female mollies. They may also show aggression if the water parameters are incorrect for extended periods. But under ideal conditions, black mollies are peaceful.

Generally, mollies are considered peaceful creatures, which applies to most of the subtypes in this category. However, it is essential to mimic the conditions your fish gets in the wild. Otherwise, even the most docile creature will start acting out.

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If you caught your molly attacking and bullying other fish, your first step should be measuring the water parameters. If they are wrong, your fish will get stressed and will act aggressively towards its tankmates. 

You should also ensure that your tank isn’t overcrowded and that there are enough hiding spots. If all the effort didn’t change your fish’s behavior, the final step would be isolation. Place your bully in a breeding tank, so it gets used to its environment without being able to attack.



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