This post contains affiliate links.
Mollies aren’t famous for aggression, but sometimes they do show this characteristic.
Mollies can show aggression towards other fish and each other. There are several reasons for that including stress and the fight for dominance.
Mollies are normally playful. If you find them chasing each other, it’s just play. But it can sometimes become a problem.
Are Mollies Aggressive To Other Fish?
Mollies may or may not be aggressive depending on several reasons such as water and tank conditions, compatibility to these other fish, or territorialism.
To find out if they’re just being playful or they’re causing trouble, we’ve listed down some signs below:
Evidence #1: Other fish are afraid.
All fish in your community tank should be healthy, active, and feeding.
When there’s bullying occurring, you will see other fish cowering when the culprit passes by. These other fish may hide, stay at the bottom (when they normally don’t), or refuse to eat. They may become skinny and lethargic.
When this happens, you should mitigate right away with the solutions we list below.
Evidence #2: There are bruises and marks.
When there’s physical evidence of fighting, heed the warning bells. Separate either the bully or the hurt fish right away.
Injured fish can become sick due to infection. While nipped fins can grow back with care, severe wounds would need methylene blue (an effective disinfectant) or antibiotic treatment.
Why Do Fish Chase or Hurt Other Fish?
Reason #1: They want to show who’s dominant.
Many animals want to show their dominance in their group. This includes fish. The winner becomes the alpha, which we fondly call the tank boss.
It even shows during feeding time when the boss would chase away other fish because he has to feed first and everybody else can just feast on the scraps.
If you’d notice, these dominant fish don’t intend to hurt the others, just chase them away. This wouldn’t be a problem if they were in the wild as the aggression would spread out in a very large group.
But in an aquarium with only 10 fish, the chances of the tank boss hurting others is higher. And if you have a small aquarium where there’s little space to swim to and fewer plants to hide behind, the problem is compounded.
Solution: Mollies are not schooling fish, but they do tend to shoal as they like to interact socially. It would, therefore, be beneficial for them to be in a group of higher numbers so their aggression would be evened out.
You can also try to put in more plants so there are more places for the other fish to hide in and break the line of sight of the aggressor. Plants also help calm down stressed fish.
Reason #2: The males want breeding rights.
It’s a primitive rule: he who has the might gets the right.
Females prefer to breed with stronger males with better genes. This leads to males fighting to get the right to breed. They do this to, again, become the alpha male. It also serves to show off the male’s prowess.
Solution: Keep a ratio of 1 male to 2 to 3 females so that there are enough females to go around. The females won’t be harassed as much, and the males can spend their energy effectively and lessen that aggression.
Reason #3: They’re defending their territory.
This is probably the most common reason why fish fight.
Mollies become aggressive when their territory is encroached. They may even fight each other in an overcrowded tank because they don’t function as a school.
Solution: Unless other fish are getting hurt, there’s nothing you should do but let the fish be. It’s but natural for them to want to defend what they consider to be their space in the tank.
If injuries are becoming prevalent, however, your tank size is probably too small to accommodate all your fish. Consider switching to a bigger aquarium or reducing the number of inhabitants in that tank.
Again, plants are a perfect fit in this situation. There will be less aggression when the other fish can hide easily from the bully.
Another solution is to rearrange the tank to confuse the fish and let them find their own new spaces. Worse cases would have you giving the bully a time out or taking him out of the tank altogether. If you want to know if a Molly can survive alone, read about it here.
Reason #4: The males want to mate.
Male livebearers like Mollies have a lot of pent-up sexual energy. This is prevalent during mating season and will rarely result in injury. But they can chase females around until they’re stressed and exhausted, and that might lead to bigger problems like disease.
Solution: There’s nothing you can do when nature takes over. As long as there are both males and female Mollies in the tank, they will mate.
Keeping that correct male-to-female ratio should solve the problem. You can even do 1 male to 4 females if you like. When you already have a lot of females, the males should be busy with mating enough to relax.
But if you don’t want them breeding, you may want to separate the males from the females. There’s more information about that here.
Reason #5: They’re not compatible.
There are general guidelines on how to choose tank mates for your fish. You can read about that in our post here.
Mollies do just about fine with lots of other kinds of fish, especially those with the same temperament. These include Corydoras, Danios, Platies, Endler’s, Guppies, Gouramis, Rasboras, Siamese Algae Eaters, Tetras, Minnows, Bristlenose Plecoes, Swordtails, and other peaceful tropical fish including other Mollies.
Don’t put Mollies with Goldfish, Bettas,
Solution: If you’ve found out that your fish shouldn’t go together and are showing aggression towards one another, rehome one of the species.
If you can’t afford to maintain another aquarium, ask a friend or your pet store to get them off your hands. It’s better to say goodbye and know they’re alive than to have them die in your tank.
Reason #6: They’re stressed.
Mollies will become aggressive towards other fish when they’re stressed. Other than the reasons listed above, Mollies can get stressed when your water parameters are not to their liking.
Solution: To find out the right conditions Mollies prefer, you can read about it in our post here. Do a partial water change right away upon confirmation.
We always advise readers to have a master test kit handy because it’s still the gold standard when it comes to accuracy. At the first sign of trouble, test your tank water.
Reason #7: They’re hungry.
This may seem far-fetched as most fishkeepers we know tend to overfeed instead of underfeed.
However, it’s still possible especially for overcrowded tanks. Overstocking is a serious problem for you and your fish as this results in constantly dirty water and territorial fights among your fish.
Solution: Other than feeding them more, you can get a bigger tank and separate some fish.
What We’ve Learned
When Mollies become aggressive, they may be due to things you may or may not be able to control. However, there’s always something you can do to resolve the situation. Don’t wait until other fish are dying before you do something about it.