Are Mollies Aggressive? All You Need to Know

In normal conditions, mollies are peaceful and require low maintenance. However, some triggers can make your mollies aggressive.

If your mollies are fighting and chasing the other fish in your tank, you may be wondering, “Are mollies aggressive by nature?”

We’ll tell you everything you need to know in this article. Stick around.

Are Mollies Aggressive By Nature?

No, mollies aren’t aggressive by nature. In fact, they’re peaceful, playful, and low-maintenance.

That said, mollies can harass and display aggression toward other tank mates, as well as other mollies, when they’re triggered.

This aggression manifests in the form of bruises and marks on the mollies’ tank mates.

Yellow Molly fish

What Can Trigger Aggressive Behavior in Your Mollies?

Mollies tend to adapt to change very easily and any beginner will be able to take care of them. However, some conditions can make your mollies stressed, and being under excessive stress can make them show aggressive behavior.

Here are some of the things that may trigger a molly:

Poor Water Conditions

The first thing to check is the water’s parameters. Mollies are highly sensitive to nitrite, so it’s advised to measure nitrite and ammonia levels regularly.

Next, check the water temperature and pH levels. A big shift in them can be stressful for your fish. Following that, assess the oxygen levels and make sure the filter and heater are working properly.

Your Tank Might Be Overcrowded

Mollies love to play around and chase each other. Having no space to do that can be stressful.

Additionally, a crowded tank means extra waste for the bacteria to get rid of, which sometimes can accumulate! That leads to a rise in ammonia and nitrite in your tank.

To help calculate your aquarium capacity, you can use a fish stock calculator.


Mollies are usually social and can live peacefully with a lot of other species. Also, mollies prefer schooling. It’s more comfortable for them to be in each other’s company, making it less stressful for them.

Being in a group can encourage mollies to be more sociable and accepting of other fish species in the tank. They feel safer together.

With that in mind, it’s advised that you keep your mollies in a group of four or more. Just make sure the females are more than the males.

Yes, mollies can survive in smaller groups or completely alone, but they might get stressed and show aggression, so it’s better to group them.

It’s worth mentioning that you shouldn’t keep mollies with other aggressive fish species. They’ll fight back!

Not Enough Food

A lack of food can be stressful for any fish, and mollies are no exception!

If the food is available in inadequate quantities, your fish will fight each other for it. So, it’s highly recommended to feed your mollies once or twice a day.

If you tend to forget to feed your fish on time, buying an automatic fish feeder is a good solution for you.

Silver Crescent Molly

Mating and Aggressive Male Mollies

Male mollies tend to be aggressive and harass female mollies during the period of mating; that’s a normal part of their mating rituals!

Usually, the male molly chases the female around the tank, which might look aggressive, but no fish gets harmed in the process, and this phase ends when mating is finished.

This phase can be avoided if you have more female than male mollies in your tank. It’s advised to have three to four female mollies for every male.

If you have a lower number of females, the males tend to fight with one another for the available females. This situation can also lead to overall aggression in your tank, where the males fight to be the “alpha” and the one in charge or even fight for territory.

That’s another reason to make sure your tank has enough space for all the fish you have.

Are Mollies Fighting or Mating?

Identifying the gender of a molly is easy. Males have a pointed anal fin, a large dorsal fin, and a smaller overall size, whereas females have a rounded anal fin, a smaller dorsal fin, and a larger overall size.

If you see a male molly chasing a female molly, it’s almost certain that it’s mating time.

You’ll also notice the males splitting before the mating process and giving unusual attention to the females around them. They’re looking for a suitable mating partner, after all. The male can keep chasing the female around the tank until he completes his mission.

If you notice a molly biting another fish or cornering it, that’s not a sign of mating; it’s probably a fight!

Pregnant Female Mollies

Pregnancy is a stressful period! Pregnant females tend to hide in a warm place and show signs of aggression toward other fish. That’s completely normal and will end after delivery.

The pregnancy period can last 20-40 days. You can tell that a female fish is pregnant simply by spotting a swollen belly.

Dull/Boring Aquarium

While it’s not mostly the main reason, it’s an important factor to consider.

Mollies are playful fish. Plants, caves, and other ornaments around your aquarium can have a positive impact on their stress levels and help keep them active. Plants can also act as hiding places for fish feeling unsafe.

These elements also act as a distraction and a sight barrier, leading to less aggression around your aquarium.

To Sum Up

Mollies are usually peaceful and social. However, some triggers can lead to aggressive behavior showing on your mollies.

You can avoid those situations by following these steps:

  • Make sure your water is always in the ideal condition.
  • Don’t overcrowd your tank with fish.
  • Add mollies in groups; one male for every three to four females.
  • Don’t keep mollies with aggressive fish species.
  • Feed your mollies 1-2 times a day.

Male mollies tend to become aggressive when they require mating; they’ll chase females around the tank but they won’t harm them in any way.

In addition, pregnant females might hide and show signs of aggression during pregnancy, which is totally normal.