Molly Fish Tank Setup Requirements


Molly Fish Tank ​​​​Setup

Molly Fish are a common sight in household and professional aquariums.

balloon molly 3

They are a small, yet tough fish breed that belongs to the “livebearer” fish group. Livebearer fish breeds are fish that give birth to live offspring rather than producing eggs.

While Molly Fish may be small in size, they can be rather demanding when it comes to their everyday care.

Knowing what Molly Fish require on a day to day basis is an excellent way to start your Molly Fish aquarium and will also make for a very enjoyable and fruitful hobby.

Most species of Molly Fish require the same types of food, habitat, and water temperature.

 However, some Molly species may be slightly more finicky about their care. It is important to know and understand this information before purchasing equipment or fish.

What is the best tank size for Molly Fish?

Many people may look at the size of Molly Fish and assume that they do not require a large tank.

question mark on black board

Unfortunately for the fish that are purchased by these owners, this is incorrect information.

A fish tank size is not determined by the size of the fish because most species of fish will continue to grow until they die.

 Some grow at slower rates than others and therefore stay small longer. Molly Fish are also not a “lone wolf” kind of fish.

They enjoy the company of similar types of fish as well as other Mollies.

Small tanks do not allow fish to have enough space for swimming, hiding, and most importantly, breathing!

 Fish breathe the oxygen found in their water. The less space the tank provides, the less water it holds, and therefore the less oxygen the fish will have.

Small tanks will also become a problem as they will become polluted by fish waste and various other bacteria much faster than a larger tank would.

All of the above facts are reasons to avoid purchasing small tanks for multiple fish.

Ever wonder why your fish, die quickly?​

fish skeleton

It is most likely the sad result of one of the above-listed problems that inexperienced fish owners, are known for.

Molly Fish are prone to various health conditions that are caused by the sudden fluctuation in water quality.

 These sudden fluctuations in water quality are far more likely in smaller tanks and tanks that are over populated.

Therefore, it would be highly recommended to start looking at tanks that are about 20-gallons for any fish, including Mollies. 

Molly Fish should also be kept in pairs. In other words, regardless of how many Molly Fish you plan to own, make sure that it is an even number!

There should also be a balance of male and female Mollies.

If your Molly Fish are all male or all female then the chances of bullying, injuries, and possibly even death are far greater than if the aquarium is co-ed.

The rule of thumb here is two females per male.

neon orange molly fish

While a 20-gallon aquarium is a great place to start, most species of Molly Fish would thrive in something larger.

 This is especially the case if you plan to own a more significant number of Molly Fish or if you wish to mix Molly Fish with other fish breeds.

If you have trouble figuring out how many Molly Fish you should purchase, there is a rule of thumb that may be helpful.

 If our tank is 30-gallons, purchase up to 6 Molly Fish. However, most fish would be happier if their owners followed the “10-gallons per fish” rule of thumb.  

Alternatively, most expert fish hobbyists will recommend purchasing the largest tank that you can afford (and have room for) and then figuring out how many inhabitants you can have.

Bottom line:  if you want you fish to thrive, give them room to survive!

How many Molly Fish can live in a 20-gallon tank?

As previously mentioned, the 20-gallon aquariums are a great place to start.​

aquarium with fishes

If a 20-gallon aquarium is the largest tank you can afford, then you should plan on purchasing up to 4 to 6 Molly Fish.

You may also be able to add a couple of snails or bottom feeder fish in the tank. However, you should take caution not to overcrowd the tank.

For example, if you can afford a 20-gallon aquarium then you should not fill it with 10 Molly Fish.

The proper way to own fish would be to allow enough space for each fish to swim, mate, hide, play, and breathe without being surrounded by other fish.

 Too many fish in the tank will also cause various health problems, increase the possibility of bullying, as well as increase the number of early fatalities.

You should also remember that the more fish you own, the more fry you will have to care for once the fish mate (and they inevitably will).

It is important to mention that if you desire a large quantity of Molly Fish in a single tank then you should purchase a larger tank or, choose the Short-finned Molly Fish as they typically stay small and their fins take up less space.

dalmation mollies

 On the other hand, this does not mean that owners can over populate their Molly Fish tanks with Short-finned Mollies.

Molly fish also require plenty of live aquatic plants to munch on as well as hide behind.

Without these plants, bullying would increase as would the danger to the pregnant females.

A 20-gallon tank should also have these plants. However, in order to house aquatic plants, you may end up cutting your guest list.​

For example, a 20-gallon tank with sufficient aquatic plants should house at most 4 Molly Fish.

Is a filter required for a Molly Fish tank?

Filtration systems are necessary for aquariums.

It is their job to remove any excess food particles, decaying substances, physical debris, dangerous chemicals, as well as fish waste from the tank water.  

A filtration system is always needed!


This is a very common mistake among fish owners. Fish, like the Molly Fish, make waste. A lot of it!

Their waste is then excreted into their water. Their water is their source of oxygen and is the main component in their everyday lives.

Imagine being forced to breathe in your waste! Aquariums without filtration systems are forcing their inhabitants to do just that and more!




Filtration systems are also in charge of removing visible debris from the water.

 Physical debris can cause the water quality to decrease, as well as potentially hurt the tank’s inhabitants. The mechanical media in the unit will take care of these issues.

Bacteria buildup is also a potential problem in fish tanks.

While some bacteria and algae are considered “helpful” to the overall environment for the fish, there is a level of bacteria that is considered “harmful.”

bacteria blue

Dangerous bacteria buildup will certainly decrease the water quality, cause cloudiness as well as cause your fish to become ill and possibly lead to fatalities.

The biological filtration media in the filtration system will help maintain the level of bacteria growth to be “beneficial.”

Finally, our water, especially tap water, is treated with various metals and chemicals to make it safe for us to drink.

 These metals and chemicals are very hazardous to fish! They cause numerous illnesses and will cause the fish’s gills to feel as though they are burning with every breath.

Treating the water with a Molly Fish safe water treatment will protect your Mollies from harm.

Also, making sure that the filtration system that you choose includes a chemical filtration media option will also help control the chemical levels in your fish’s water.

Some filtration systems are stronger than others, and some may have a large opening.

clean a fish tank

If your Molly Fish can fit inside the filtration system’s opening, then you should consider a different type of filtration system.

Alternatively, you can place a mesh over the opening to prevent the Molly fish from being sucked into the mechanism, while still allowing particles to be removed.

Another aspect to keep in mind is that some species of Molly fish, as well as other compatible fish breeds, may be overwhelmed by a powerful filtration system.

For example, the Balloon Molly Fish and the Potbelly Molly fish have a more rounded body shape.

 This body shape may affect their swimming abilities.

Slower swimming fish (such as these two Mollies) could have a difficult time swimming properly in a tank with a powerful current caused by the filtration system.

Choose the best filtration device based on your tank’s inhabitants.

Is an air pump necessary for a Molly Fish tank?

An aquarium air pump is a unit that is designed to use pressure to move air in an aquarium. They help increase the oxygen levels in the tank water.

Mylivell Quietest Aquarium Air Pump Ultra Silent High Energy Saving Oxygen Air Pump Aquarium for Fish Tank with Air Stone and Silicone Tube Black

Without an air pump, the tank water may lack sufficient oxygen for it’s’ inhabitants to breathe.

Air pumps are needed for some fish tanks. However, most Molly Fish experts seem to think that an air pump is simply an excellent additive for a Molly Fish tank.

Air pumps are beneficial to the everyday life of a Molly Fish.

Air pumps create bubbles which then rise to the water’s surface.

This causes what is known as “surface agitation.”

Surface agitation comes in handy because it produces more oxygen in the water for the fish and other inhabitants to breathe.

Another benefit of installing an air pump is that you can attach a sponge filter on the end of the pump itself.

This sponge then acts as a secondary filtration system.

 It adds to the clarity and overall quality of the water when it is working alongside another filtration system, and it can serve as a temporary filtration system should the main one fail.

 Added filtration and increased oxygen in the water are never bad things when talking about the overall health and well-being of our fish.

pregnant gold molly fish

However, an air pump is not required in a Molly fish aquarium.

Is a heater needed for a Molly Fish tank? If so, what is the ideal water temperature?

An aquarium heater is a device designed to increase the temperature of the water inside the tank. Once the tank water is at the appropriate temperature, the heater works to maintain that temperature.This is an important device as many fish breeds require a specific water temperature to survive and drastic fluctuations in water temperature could be harmful to the tank’s inhabitants. 

Molly Fish tanks do require an aquarium heater.

Submersible Aquarium Heater Explosion-Proof Auto Thermostat Fish Tank Heater with Protective Sleeve & Suction Cup, Digital Temperature Readout, 300W

Aquarium heaters come in various brands and sizes. You will need to purchase one that is designed for the tank you purchased.

For example, if you chose a 30-gallon aquarium then you will need an aquarium heater that can handle up to 30-gallons of water.

Molly Fish thrive in warmer water temperatures.

This explains one of the reasons why Molly Fish are not typically tankmates with Goldfish as Goldfish prefer slightly cooler water temperatures.

Some aquarium heaters (as with various other aquarium equipment) are made of a cheap plastic material. It is vital that you choose a material that is durable and will last.

Cheaper materials are more prone to breaking and malfunctions which could cause problems for your fish.

Molly Fish are considered a tropical fish species and would prefer their water temperature to remain between 75ºF and 82ºF.

Water Temperature
Water Temperature

Any drastic change in water temperature could cost your Molly Fish their lives! Take caution when adjusting the water temperature and do so slowly.

Molly Fish can be relatively simple fish to care for if you are an experienced fish owner. However, Molly fish can also be rather time-consuming and demanding.

This is due to the fact that they have specific living requirements that must be met in order to thrive in their habitat.

Most first-time fish owners will make common mistakes that will, unfortunately, cost the Molly Fish their lives. Many of these common mistakes are included above.

Many of the various Molly Fish species have similar characteristics and features which could help you create a diverse environment in your aquarium.

platinum molly fish swimming

However, it is important that owners observe Molly Fish and choose them based on similar personalities and temperaments.

This will reduce the amount of injuries, bullying, and possible fatalities that are common when Molly Fish are aggressive.

Regardless of the Molly Fish species you choose, you should make sure that you are providing them sufficient care and attention that they deserve!