Molly Chasing Another Molly – Reasons

By Nadine Oraby | 2020 Update

Molly fish are rarely aggressive and in most scenarios, they happily swim back and forth without bothering their tank mates. However, one day you find one of the molly chasing and nipping at the other. To any responsible aquarist, this can be a worrying sight because often the ‘catfights’ between fish can also become nasty.

Why are your mollies chasing each other? There are many reasons for the mollies to chase each other. Predominantly, a male molly might chase its female counterpart to mate. A male molly may also become aggressive with the other male population in the tank to mark its territory or for the attention of females present in the aquarium.

Boasting a delicate and attractive aesthetic, the molly fish are one of the most common types of aquarium fish. They are adaptable and hardy, making an ideal pet for even the most novice fish keeper. However, their aggressive nature is a challenge and providing them with the best tank conditions is the best way to ensure peace.

Fighting or Mating – Understanding Aggressive Behavior

Before determining the reasons for the fight between mollies, it is first important to know the gender of the fish in tank. Here are some ways to differentiate between them:

  • Female mollies are bulkier while male are more torpedo-shaped
  • The males have pointed gonopodia
  • Females are wider from the anal side for breeding
  • The males have larger fins while the female boast short fins with triangular-shaped anal fins
  • The male mollies have vibrant coloring than the female

Once you are aware of the sex of your molly fish and if you find the male chasing the female molly, chances are he is trying to show off and encourage mating.

However, make sure you have a 1:2 ratio of male to female if you are planning to breed the mollies. If there are many males and only a few females in the same tank, the males will fight each other to chase a female.

The female molly can also retaliate and start doing the same to the males in the tank, as she will get tired from the attention she is getting. In many occasions, the frequently targeted mollies are injured while trying to save themselves that can result in death as well.

Male vs female molly

With mollies, it is important to establish the perfect gender mix because it has a significant effect on their behavior. Mollies are very hierarchical within their genders and if multiple male mollies are placed together, there will be episodes of fights to establish who the boss of the tank is.

In rare cases, the ‘fights’ can be a sign that you have an overly aggressive fish. The fish have personalities like us and some are more hostile than the other members of the tank.

Is the Molly chasing the fry?

Mollies are livebearers meaning they give don’t lay eggs and their fry comes swimming out with their siblings.

Unfortunately, adult mollies lack parental instincts and may chase the juvenile to eat them. In fact, they can be even eaten by their own parents – as gruesome as it may sound.

To survive, the fry needs some help from you. Here are some tips to safeguard them:

  • Separate the pregnant fish from the community tank and put her back after she gives birth
  • Invest in a breeding box
  • Put up large plants in the tank to provide hide ways

Baby fish grow to be adult size within 1 – 2 months. As a general rule, the young fish should be larger than the mouths of adult fish.

Do not move the molly fry to the community tank until you are sure they are ready to handle the adults.

Adequate tank and food conditions

Is your tank overcrowded? Is it too small and providing less mobile space to the fish? Is the water ammonia level correct? Are you feeding sufficient amount of food that is distributed equally amongst the tank mates?

Unsuitable aquarium conditions can put stress on the fish and might make them aggressive towards each other. Check your water conditions regularly and conduct changes as required. Ensure the food supply is ample for all the fish in the tank as well.


Aggression is a common trait of fish – regardless of their type. However, the level of negative nature can be diminished by taking some extra precautionary measures. `

Try the above tips and see if they work in reducing aggression. If nothing works, the fish may be a “bad guy” and possess a violent streak. The best way to ensure the safety of other fish from the dominant one is to remove the aggressive fish from the community tank.

Good luck!

Related Questions

Molly fish are compatible with? The mollies are generally social and may not like living in isolation. Besides their own kind, some good tank mates for mollies are swordtails, platies, corydoras catfish, angelfish, and some species of tetras.

What is the gestation period for molly fish? The average gestation time of molly fish is 60 days and may release 10 – 60 fry at the same time.

Moreover, the mollies have the ability to store sperms for months and may reproduce every 30 days after the first delivery.


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