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are molly fish aggressive

Are Molly Fish Aggressive? How to Stop it.

Molly fish are probably one of the most popular choices for aquarium owners both new and experienced. They are lovely to look at, peaceful, and full of character.

One of the best things about owning a community tank is having a wonderful array of different fish, covering different shapes, sizes and colours, as well as getting to see their personalities take shape as they get used to their new home.

There is a downside to having a community tank though. Sometimes trouble brews between different species, or within a species.

There is a way to eliminate this though, which is to do your research before adding a new type of fish to your tank. This way, you will spend less time stressing over the stress levels of your fish kids, and more time enjoying how they all get along peacefully together.

In this article, I’m going to discuss if molly fish are aggressive, what can cause any aggression, how to recognise it, and how to reduce it.

Are Molly Fish Aggressive?

Mollies are not usually known for being aggressive to other species. However, they are capable of showing mild aggression to other mollies if they feel threatened, or are establishing a pecking order amongst themselves, especially if new fish have recently been added to the tank.

molly fish
A pair of molly fish.

Reasons Why Mollies May Show Aggression..

Establishing a pecking order.

This is relatively normal behaviour with mollies if you have more than one male. The group needs to establish who is the boss and who isn’t. This is usually decided by very low level aggression, which can often be mistaken for playing. It can get a bit more aggressive if some of the males refuse to back down though.

Not enough space.

Mollies need space to swim, as they love to cover most of the tank. A tall tank is also good for them and any other active fish as it gives them more swimming space.

Use a stocking calculator, like AQADVISOR.COM to help you work it out. They take the shape of your tank into consideration, as well as the size.

Not enough food.

It’s not just humans that get hangry. A hungry fish can be a very aggressive fish. Not feeding your fish enough is as bad as overfeeding them. Getting the amount right for your tank can take a few goes to get correct, but it is very important.

They are in spawning condition.

When male mollies are in spawning condition, they will have a go at other male mollies if they annoy them. The male’s temperature increases when they are ready to spawn, and we all get a bit irritated when we’re too hot, don’t we? The male will basically chase his female partner constantly when they are getting ready to spawn. It can look like he is looking for a fight, but if you look closely at the other fish, you should see she has a more rounded belly than normal.

Not leaving the ladies alone.

I have noticed this with my mollies. The males are constantly chasing the females around in quite an aggressive manner. This is why you should always try and have 3 females to every male. Even then, the females are still being hounded constantly. Molly fish do not form pairs. Male molly fish are not that fussy.

I have a few feisty females in my tank who will tell the males off when they are bothering them. Not that they pay much attention to it though…

molly fish
Molly fish.

How do Mollies Show Aggression?

There are other reasons why mollies look like they are showing signs of aggression, when in fact, it is not as bad as it looks.


Sparring amongst mollies is relatively normal when they are in a group with more than one male. This is how they sort out their pecking order. The 2 sparring fish will basically chase each other around the tank. I know you’re probably thinking that’s what they do all day anyway, but, if you watch closely, you will see the same 2 fish chasing each other constantly. They will repeat this until they get bored (or distracted).


When molly fish are mating, it looks very similar to when they are being aggressive to each other.

You can tell which of your females by looking at their shape. Male mollies tend to be slimmer. They also have slightly more vivid colours than the females.

black molly
A Black Molly Fish.

Here are a few things for you to look out for, to help you decide if your mollies are actually being aggressive, which could threaten the health or well-being of your other fish.

Signs of aggression to look out for.

  • Their fins spread, or stand up rigid, rather than flowing in a relaxed manner.
  • Enhanced colouring.
  • Darker eyes.
  • Twitching head.
  • Skittish behaviour

How to Reduce Aggression in Mollies.

Correct male to female ratio.

I would always recommend having your male to female ratio as 1 male to every 3 females at the very least. This will stop the females from being harassed constantly by the males.

Correct tank size.

This is essential no matter what types of fish you have. One of the main reasons why fish get stressed is because they don’t have enough space. This can result in fish hiding away, or fighting with their own type, or other breeds, because they feel trapped.

Always make sure you don’t overstock your tank. When calculating how many fish can fit into your tank, you have to calculate using their adult size, not the size they currently are. Always try and understock slightly to be on the safe side.

As well as considering the size of your tank, you also need to think about the shape of it as well. This is essential when it comes to fish like Danios, who are one of the most active fish to put in your tank. They need plenty of space to dart around in.

Have hiding places.

Having some ornaments and plants in your tank is always a good idea. You don’t have to have real plants either, the imitation ones do their job in providing shelter or hiding places for fish for when they need to just have a moment to themselves.

Fish can feel exposed when in a stressful situation so it is good to have somewhere for them to retreat to. Having a hiding place or a bit of shelter makes the fish feel a lot safer.

Just always make sure that you don’t fill your tank up with things so much that your fish have no room to swim and have fun, as that can be just as stressful as having nowhere to hide.

Keep the correct number of Mollies.

Mollies are more peaceful when in a group of at least 4. They thrive and are happiest in groups of around that number. Don’t have too many more in your tank (unless your tank is massive) because that can lead to aggression over hierarchy.

Putting a single molly into a tank is never a good idea as they can become quite defensive. This will most likely lead to aggression with the other fish.

molly fish
Somebody didn’t get the memo about numbers..

Feed them the correct amount.

When you are new to fishkeeping, you are always told that you should never overfeed your fish. It is just as important that you don’t underfeed them as well, as that can also have a damaging effect in your tank.

Hungry fish can be aggressive fish as they may have to race to get to the food first if you aren’t feeding them enough.

It really is a case of trial and error at first when it comes to feeding your fish the correct amount.

A rule of thumb is that all the food you feed your fish should be eaten within 3 minutes. If your fish finish eating everything well within that time, you are not feeding them enough. If, after three minutes, there is still a lot of food floating around the tank, you are feeding them too much, so remember to adjust what you give them the next time you feed them.

Also, well fed mollies are less likely to eat their fry, which is another thing to take into consideration if they were to have baby mollies.

Get your timing right.

It is best to add your molly fish to the tank at the same time, and preferably when they are juvenile.

If you were to add mollies to a tank that already has some in it, this could cause a problem. The fish in the tank will already have sorted out their pecking order so could be a bit miffed at new guys being added.

Having said that, I have added 2 new mollies to my tank a few months after my original ones and I didn’t see a problem, apart from the new males chasing the females straight away.

Best Tank Mates for Mollies.

A popular choice of tankmate for mollies are tetras or rasboras, especially the ones that aren’t easily picked on. They tend to just hang out in their own little group, but are just as active, so won’t be freaked out by all the movement in the tank. Other mollies are also fine as tankmates. In fact, the more the better, as long as you keep to the recommended male to female ratio.

molly fish
Lemon and white coloured molly.


Mollies always make a great addition to a community tank, provided you don’t overstock your tank by adding too many fish in.

Male mollies can show aggression to other mollies, but this is normal behaviour and nothing to worry about.

Related Articles:

Do molly fish change colour?

Why is my molly staying at the bottom of the tank?

Is my molly a bully?

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