Should Mollies Be Kept in Pairs?

One of the most popular aquarium fish is mollies. They are tough little fish, yet they do need some special care. Any aquarium enthusiast is off to a good start when they learn what molly fish need for their everyday upkeep.

While there are countless varieties of mollies, they all need the same kinds of food, water temperatures, and habitat. Some species are a bit fussier on certain things, so knowing this is important to be sure you have harmony in your fish tank.

There is one thing new tank owners are not always aware of, and that is how many mollies they can keep in a tank to keep a balance. Adding to this, there is another question that may not appear relevant.

Therefore, should mollies be kept in pairs? Yes is the simple answer to this, and it doesn’t matter how many mollies you plan on owning. You always need to have an even number. To stop this, there should also be a balance between males and females. This balance isn’t one to one and is one male for every two females.

One of the biggest issues here is that mollies are livebearers, and can give birth to lots of fry repeatedly. Because of this, you can find your tank overrun with mollies, and attempting to keep them in pairs can be an almost impossible task.

Read on, and you can learn most of what you need to know about keeping mollies in pairs, and in harmony.

How Many Mollies and What Sized Tank?

An average-sized tank for new aquarium enthusiasts is around a 20-gallon tank. For mollies, you should select this as a minimum. Tanks of this size can accommodate either four or six fish. If you want other wildlife, you can add a couple of snails or fish, which are bottom feeders. These can help keep your tank clean and won’t interfere with your mollies.

However, you should not think you can trade these bottom-feeding fish and snails and top up the numbers of mollies to ten, because this won’t work.

The correct way to provide for fish is to give them plenty of space to swim, play, breathe, and mate. Although mollies are shoaling fish, even they don’t like to be overcrowded. Once this happens, you find there is bullying and fatalities can happen as a result.

One final thing to note is the number of fry you may have to contend with unexpectedly.

If you do wish to have more mollies in the same tank, and can’t afford a much larger tank, then you can opt for one of the short-finned types, although these are not as nice to watch, and it doesn’t mean you can overcrowd these.

Mollies are partial to munching on live plants now and again; besides, this can help prevent bullying to pregnant females. While this means you can add plenty of these for their snacks, and for them to hide in. There is one final issue here, and that is if you have a large number of plants, it can be too much for your 20-gallon tank, and you have to reduce the number of fish accordingly.

Setting Up Your Tank for Mollies

It is unlikely you will have mollies that come from the wild, and the ones you get will be bred in captivity. You can find this is a benefit, as they will already be acclimatized to a home tank setup. When setting up your tank, one of the first things you should aim for is a sandy substrate.

You can use this on its own, or you can use it to cap a soil under substrate that will help your live plants grow. Mollies won’t be spending much of their time in the sand, yet it makes a great footing for your wildlife.

Because your mollies will be in the upper parts of the water, you need taller growing plants such as Anubias nana. This is ideal for delivering a great shelter for your fish. Apart from these, you can add rocks you can make into caves and crevices.

Spaces like these deliver ideal areas where fish can escape if they are being picked on, or they want some alone time.

The next addition to your tank needs to be your heater. Mollies and other tropical fish like temperatures in the range of 72-78 degrees fahrenheit. Other areas, which need to be in specific ranges, are pH levels that are between 6.7 and 8.5, and a water hardness of 20-30 KH.

If you are purchasing a tank that comes with lighting, this will be more than likely sufficient.  You can say the same for your tank filter. If this comes with your tank, it can be sufficient for your tank, as your mollies will love the gentle flow your filter delivers without additional air or water pumps. Your filter though must contain a chemical filtration media option, so be sure to check this before purchase.

New enthusiasts think they can use plain water from the faucet. This contains chemicals that can be toxic to fish, so it is best to treat it before use. If you use water that isn’t treated, it can make fish feel as their gills are burning with each breath they take.

Air pumps are not necessary for a tank, which has sufficient agitation from the filter, although the addition of an air pump can make all the difference to the condition of your tank.

The more oxygen in your water, the better it is for your fish, your plants, and the overall condition of your aquarium.

Breeding Mollies and How to Control Them

One issue with livebearers like mollies is they tend to breed easily. You may begin with four mollies and soon end up with twenty.  

In many instances, mollies take care of this issue themselves, as mollies are omnivores and opportunistic eaters. They happily eat fry of other mollies and their own.

However, when you have a large number of plants or other hiding places. There is the chance a few can survive. Because of the ease they give birth, you should always be using a larger tank than is seen as the minimum to cope with this scenario.

If you end up with molly fry and want to keep them alive, here are some ways to do it:

  • Isolate the fry. If you can, move the pregnant molly to a separate tank, or you can use a breeding box. This means you can separate the fry from the mother, and the rest of the tank. Once they are large enough not to be eaten, you can add them to your tank, or pass them on to another tank or sell them.
  • Increase the tank temperature. Higher water temperatures (such as 80F) help speed up the metabolism of mollies and cause them to grow larger faster, which means less time until they are large enough to handle themselves.   

You can easily tell the difference between males and females. The anal fin on each varies in size and shape.

  • Males: The anal fin is long and thin in the shape of a stick
  • Females: Anal fin is more fan-shaped

When your mollies give birth to fry, you can feed them the same food, though this does need grinding into a powder. You can also grind bloodworms and brine shrimp to feed them also.

Once your fry is old enough, and you can see the difference in sexes. It is time; you need to begin separating them from each other.

The reason for doing this is, so they don’t begin mating straight away. At around eight weeks old, you should have all your male fry away from your female fry, because this is where they reach their sexual maturity.


Mollies are very sociable fish, and the speed they mate, it appears they can be over sociable. Keeping mollies in pairs is done for a reason, and it is a good way to help any enthusiast to keep on top of how many fish they need to keep in their tank.

As we saw, a 20-gallon tank is ideal for three mollies, of which two will be female and one will be male. This isn’t exactly a true paring, yet each female can pair with the male.

Mollies are a great addition to any tank, and once aquarium enthusiasts understand what they need to care for them, and how to spot the signs of pregnancy, they can reduce the chances of fry quickly overcrowding their tanks.