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How Many Molly Fish In A Tank? (1-75 Gallons)

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One of the first questions that many people ask when starting a molly fish tank is how many of them should be in a tank. As with any hobby, it’s essential to do your research before investing in livestock. Luckily, over the years, I learned some interesting facts regarding that question.

Each molly fish requires five gallons of water on average. For example, 20-gallon tanks can accommodate a group of four mollies, while 10-gallon fish tanks can hold merely two. That relies on the assumption that the average molly size is 4.5 inches long.

As we move forward, I will list how many mollies can live in your tank, from 1 to 75 gallons. Then, I will show you the proper male-to-female ratio and how to grow multiple mollies in the same aquarium successfully.

How Many Mollies Should Be Kept Together?

While mollies are fun, colorful creatures, you can only maintain their peaceful temperament by keeping them in aquariums of the correct size. People do not realize that keeping a molly fish in a small, crowded tank can make it aggressive.

When it comes to choosing a tank, you have to remember the ‘One Inch of Fish Per Gallon’ rule.[1] It is not a perfect rule, but it will give you a basic idea of the tank sizes you should target. Naturally, to use that rule, you have first to determine the size of your mollies. Molly fish have an average size of 4.5 inches.[2]

If your fish fall within that size range, this is what you can expect:

Small-Sized Fish Tanks:

  • 1-Gallon tanks can hold zero mollies.
  • 2-Gallon tanks can hold zero mollies.
  • 2.5-Gallon tanks can hold zero mollies.
  • 3-Gallon tanks can hold zero mollies.
  • 3.5-Gallon tanks can hold zero mollies.
  • 5-Gallon tanks can hold one molly.
  • 6-Gallon tanks can hold one molly.
  • 6.5-Gallon tanks can hold one molly.
  • 7-Gallon tanks can hold one molly.

Medium-Sized Fish Tanks:

  • 10-Gallon tanks can hold two mollies.
  • 15-Gallon tanks can hold three mollies.
  • 20-Gallon tanks can hold four mollies.
  • 29-Gallon tanks can hold six mollies.
  • 30-Gallon tanks can hold six mollies.
  • 36-Gallon tanks can hold eight mollies.

Large-Sized Fish Tanks:

  • 37-Gallon tanks can hold eight mollies.
  • 38-Gallon tanks can hold eight mollies.
  • 40-Gallon tanks can hold eight mollies.
  • 55-Gallon tanks can hold twelve mollies.
  • 60-Gallon tanks can hold thirteen mollies.
  • 75-Gallon tanks can hold sixteen mollies.

As was noted above, the formula is not perfect. It doesn’t account for the other objects in the tank, such as rocks, plants, and driftwood. It also doesn’t account for the various sizes in which you can find molly fish. Some mollies are larger than 4.5 inches. Others are much smaller.

The rule is supposed to give you a general range that you can use to make decisions whenever you go shopping for aquariums. For instance, it tells you that six mollies should not live in an aquarium smaller than 29 gallons.

It might be easier for new aquarists to remember that mollies require a minimum of 10 gallons. If you have two mollies in ten gallons, you need five additional gallons for every new fish you introduce to the aquarium.[3]

What Is The Right Male-To-Female Ratio In Mollies?

You need two to three females for every male molly. Even though mollies are largely peaceful, a tank with a small number of female mollies, for instance, one female for every pair of male mollies, could incite aggression in the male mollies.

First of all, because they are enthusiastic breeders, the male mollies will harass the few female mollies in the vicinity. This can lead to the death of the female, especially if a male molly tries and succeeds in mating with the same female molly on multiple occasions. The female may not survive the exhausting experience.

Secondly, the male mollies may fight one another for the females, which you do not want. Keeping large numbers of females will dilute the aggression among the males. The wide variety of mating options will keep them distracted. This will protect the females in the long run.

Can Two Mollies Live Together?

You can successfully keep mollies in groups of two. Though, you are better off keeping them in groups of four or more. That is because some mollies may become aggressive if they are forced to live with one other molly, especially if both mollies are male.

A larger group of mollies will dilute the aggression. They will not be as aggressive towards each other. A larger group will also make it easy for the mollies to find a mate and reproduce. But if you have to keep only two mollies, make sure they are both females.

Should Mollies Be Kept In Pairs?

Keeping mollies in pairs may end up poorly if you have both genders in the tank. Yes, male mollies are more dangerous when they are forced to share a small number of females. But a single male molly can also make a single female molly’s life miserable.

Admittedly, the signs of aggression that some people observe in their molly tanks are not always true signs of aggression. After all, mollies will chase each other around when they are mating. What some people perceive as aggression is just part of the mating ritual.

However, when a male molly has only one female to mate with, it may shower the female molly with more attention than she can handle. If the male molly is determined to mate multiple times over a short period, the stress and strain may kill the female molly.

You are better off keeping at least three mollies in a tank: one male molly and two female mollies. However, it is more than possible for one male molly and one female molly to live peacefully in a tank.

How To Grow Multiple Mollies In One Fish Tank?

If you want to rear multiple mollies, you must create a conducive environment to permit the fish to thrive. That includes the following:

1. Pick The Right Tank (At Least 10 Gallons)

I highly recommend getting a large tank. Mollies require at least ten gallons. A small tank is not only more challenging to maintain, but it will induce stress. Mollies in a small tank are more likely to become aggressive. If you are interested, here I included a review of the particular 20-gallon tank that I use.

2. Put Some Decorations

Mollies do not like bare tanks, which is why I suggest adding plants and decorations. Don’t crowd the tank. Take the plants and decorations into account when you are selecting a tank. A tank with too many plants and decorations is just as uncomfortable as a tank with too many fish. Keep the plants at the edges so that the mollies have plenty of open space to swim.

Mollies like decorations because they can use them to hide from bullies. Plants and decorations create a sense of security, the kind that puts the mollies at ease, preventing them from succumbing to stress.

Don’t forget to include a substrate. Because you need the substrate to anchor the plants, you need one or two inches of materials that have a lot of nutrients at the bottom before adding a few more inches of gravel or sand. The plants need the nutrients in the substrate to thrive.

3. Adjust The Water Quality

Once the plants, decorations, and substrate are in place, you can add water to the aquarium. Make sure the tank is cycled to completion. An uncycled tank will expose mollies to dangerous spikes in ammonia. Cycling introduces beneficial bacteria that can process the waste that fish produce, not to mention the ammonia that comes from the waste.

But cycling is only the first step. You have to alter the water’s chemistry to ensure that the parameters are accurate. The goal is to mimic a molly fish’s natural habitat. That means providing a temperature of 72 to 78 degrees F, a pH of 7.5 to 8.5, and hardness of 20-30KH.[4]

Here is the equipment that I use in my tank to set these parameters:

  • I got the API FRESHWATER MASTER TEST KIT (link to Amazon) to measure the pH, ammonia, nitrates, and nitrites. If any of these toxins is above 0 ppm, you’ll require more frequent water changes. Generally, I change 15 to 20 percent of the water weekly.
  • I got the 6 in 1 Aquarium Test Strips (link to Amazon) to measure the water hardness. You can also use this kit to test parameters like pH, nitrates, and hardness. That is a perfect choice if the API testing kit above is too expensive.

4. Get A Heater

Some people rely on the ambient temperature in a given room to maintain the temperature in the aquarium. But that is dangerous because it leaves the aquarium’s temperature at the mercy of the fluctuations in the ambient temperature.

As a rule of thumb, aquarium fish hate instability. This is why aquarists are encouraged to install heaters. They will enable you to maintain the stable warmth that molly fish want. Here is a review I wrote on the device that I use in my tank.

5. Choose The Right Number Of Mollies

Generally, I recommend aiming for three or more mollies. If you have only three mollies, two of those fish should be female. Though, the larger the group, the better. When selecting the mollies from a store, avoid fish that live in dirty, cloudy water with lethargic, sickly fish.

Choose mollies that do not have any marks, bruises, blemishes, folded or tattered fins, etc. Sick fish can get better once you introduce them to the healthy conditions in your home aquarium. However, sometimes, not only do they get worse, but they may infect the other fish in the tank before they die.

This is why it is vital to keep new fish in quarantine for a few weeks to ensure they do not have any diseases. If you are satisfied with the health of the new fish, take a moment to acclimate them. That means keeping them in a bag of their old water and then floating that bag in the new tank for 15 minutes.

You can also use the drip method to get them accustomed to the chemistry of the new water. Once you add them to the tank, watch them for a few minutes. If you observe any signs of distress, take them out.

  • If you are new to the drip method, here is an excellent Youtube video that illustrates how it works:

6. Encourage Breeding

If you want the mollies to multiply, make sure you have multiple females for every male. The mollies don’t require coercion to mate. Once a female molly’s stomach bulges, showing that it is pregnant, move it to a separate tank to give birth without being hassled by the male mollies.[5]

Do not move the molly fish when it is just about to give birth. The stress of the transition could cause the fish to abort its young. Once the molly fish gives birth, take it back to the main tank before it eats its young. 

I highly recommend that you raise the fry separately, feeding them brine shrimp, blood worms, and the like until they are large enough to survive in the main tank. If you’re interested, here is an article that I wrote on how long it takes for molly fry to grow.

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If you wish to keep multiple mollies in your tank, the first thing you have to consider is their size. The average size of a molly fish is approximately 4.5 inches. Following the “one inch per gallon rule”, you’ll need a tank of about 15 gallons for a group of three molly fish.

You should also take plants and decorations into account. Also, if your aquarium features other fish, you might need fewer mollies than recommended. Generally, the aquarium should seem spacious enough for your fish to swim freely.