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molly bullying

Molly Bullying: What Causes it and How to Stop It.

A community tank can be a perfect way of keeping a wonderful array of different fish, covering a variety of shapes, sizes and colours. You can spend hours watching them, 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 virtually eliminate this, 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 molly bullying: what are the causes of it, and what can be done to stop it.

Do Mollies Bully Other Fish?

Mollies have been known to bully other fish. These tend to be other mollies, or similar sized fish, and there is nearly always a reason for it, like overcrowding, no shelter, incorrect male to female ratio or not enough fish in your tank.

molly fish
Group shot!

Reasons Why Mollies May Bully Other Fish.

They are 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 one of the males refuses to back down though.

Not enough space.

Mollies need plenty of space to swim, as well as quite a tall tank so they have plenty of space both above and below them.

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.

When it comes to mollies, they can get quite aggressive with each other when it comes to food, even if there is more than enough for everyone. If you have a nice, big tank, this won’t be as much of a problem.

They are in spawning condition.

When male mollies are in spawning condition, they will have a go at other male mollies if they see them as competition. 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.

Male mollies can be a right pain when it comes to the lady mollies and will constantly chase the females around in quite an aggressive manner if your male to female ratio is wrong. This is why you should always try and have 3 females to every male.

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 guppies are mating, it looks very similar to when they are being aggressive to each other.

You can tell which of your mollies are female by looking at their shape. Male mollies look more slender. They also have more vivid colours than the females. Female mollies are not as vivid in colour compared to the males.

molly fish
Are they lovers or fighters?

Here are a few things for you to look out for to help you decide if your fish in being bullied by another molly.

Signs of Molly Bullying:

  • Damaged fins
  • One of your fish staying away from the rest of the group
  • A fish hiding away a lot
  • Not eating
  • Swimming in an unusual manner-they could be injured

How to Reduce Molly Bullying.

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.

It is also a good idea to not have too many male mollies in your tank. This will help to reduce aggression between them.

If you are not interested in breeding mollies, you may want to consider an all female group, as this will virtually eliminate any bullying.

Correct tank size.

This is essential no matter what breeds 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 guppies, who are one of the most active fish to put in your tank. They need plenty of swimming space.

Have hiding places.

Having some ornaments and plants in your tank is always a good idea as it creates shelter, as well as places of interest. 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. It also provides a distraction for any bullying fish as the other fish are not always in their line of vision.

Just always make sure that you don’t fill your tank up with so many things 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 thrive and are happiest in groups of 5 or more. Don’t have too many more in your tank (unless your tank is massive) because that can lead to aggression over hierarchy.

Try not to keep only one molly as this can make them defensive, which will then cause aggression with other fish in the tank.

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.

Get your timing right.

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

This may sound like an obvious thing but make sure your mollies all look healthy when picking them at your local aquarium. A sick fish, or a fish noticeably smaller than its fellow mollies is always a prime target for bullying.

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. If you only added female mollies at a later date, there shouldn’t be a problem.

Best Tank Mates for Mollies.

A popular choice of tankmate for mollies are tetras or rasboras. 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. Guppies are also fine as tankmates.

Avoid any fish that are known to be fin nippers (step forward, Tiger Barbs..).

black molly


Mollies can make a great addition to a community tank, but bullying between mollies can occur. However, there is nearly always a reason for this, and there are ways to prevent it from becoming a problem.

Related Articles:

Are molly fish aggressive?

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

Can mollies change colour?

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