How to Start a Molly Fish Tank? – The Ultimate Guide for Beginners

Beautiful, varied and lively, molly fish are a must-have fish that can be kept in a community or a species-only aquarium.

Mollies are prolific breeders, breeding often and producing a lot of fry, which is typical of all livebearer fish.

If you’ve set your mind on starting a molly fish tank, you may be surprised by some of the things required by molly fish. This especially considering their small size and otherwise undemanding nature.

If you want to offer your molly fish the best care and a habitat that closely resembles their conditions in the wild, in this article, I’m going to get you up to speed with the ins and outs of how to start a molly fish tank.


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Although there are some differences between the requirements of various breeds of molly fish, the things I’m going to discuss in this article generally apply to all mollies.

Molly Fish Tank Set-Up

Setting up a molly fish tank isn’t more difficult than setting up any other freshwater aquarium, but if you’re a beginner you may not know what’s required for your fish tank and what’s optional.

If you look at the size of molly fish (3-4 inches on average), you may think they’ll do fine in a small aquarium.

And while adult size can determine the size of the tank needed, there are some other factors at play that can have a significant role in determining the size of the tank needed for mollies.

For starters, molly fish are active fish that spend a lot of time swimming around in the tank, thus, they require enough space to be able to swim freely.

Secondly, molly fish have a rather high bioload, so a small tank is a hazard for them. With toxin levels rising because of the waste they produce, oxygen levels drop, and waste products build up much faster in a small tank compared to a larger one.

Other than concerns related to water chemistry, another issue with mollies is that they don’t like being kept alone; they need the company. So, keep them in groups, preferably in even numbers.

Molly fish — like most fish sensitive to sudden changes in water chemistry – are prone to diseases. Acute temperature changes and water chemistry changes are far more likely to occur in small aquariums as opposed to larger ones.

Even if you flawlessly perform the nitrogen cycle before adding mollies to your aquarium, the balance in the water can be disturbed when high amounts of toxins are released, and water changes aren’t regular, or they’re not performed at a suitable percentage.

What Tank Size is Suitable for Molly Fish?

Now that we’ve established that a small aquarium is not an optimal choice for a molly fish tank, let’s see what size would be appropriate.

I often see molly fish care guides mentioning 20-gallon tanks as the bare minimum for molly fish (although I’ve seen smaller tanks mentioned too – a huge no-no in my opinion). While a 20-gallon tank is good place to start, I’d still recommend getting something bigger.

A bigger tank is required, especially if you’re planning on housing a larger group of molly fish, or if you’re considering setting up a mixed-species tank.

To help you determine the appropriate tank size for your mollies, here are a few tips that can guide you:

– It’s always better to oversize your tank, rather than undersize it

When it comes to keeping mollies, a small tank is a hard pass – your fish will suffer, you’ll have a hard time getting the water chemistry stable, so it’s an all-around bad idea.

While you can certainly go wrong with a small aquarium, you usually can’t go wrong with a large aquarium that will offer your fish room to swim, hide, and breed, plus it will allow you to keep the water chemistry at optimal levels.

If you can afford it and you have enough space where to keep the aquarium, always go for the bigger option.

– If you can’t afford a 20 gallon+ tank, keep fewer molly fish

If a 20-gallon tank is all you can afford, don’t stock it with too many mollies. Keep up to 4 mollies, so they’ll feel comfortable and get enough space.

– Account for any potential fry that may result from breeding

It’s a well-known fact that livebearer fish breed constantly, and they produce a lot of babies. Mollies are no exception; they can breed as often as every month and they can produce up to 100 fry every time.

Fry resulting from your molly fish breeding will also take up valuable space and oxygen in the aquarium. They can also throw off the water balance by producing a lot of waste.

In the section of this article titled “How to stock molly fish?”, I offer a few tips on how to prevent molly population from getting out of hand.

– Look into molly breeds that generally stay small if getting a large tank is an issue

If a 20-gallon tank is the maximum size you can afford, look into molly breeds that generally stay small. Short-finned mollies typically don’t grow too large and their fins take up less space.

– Be very careful not to overcrowd the tank

It always bears repeating that overcrowding a fish tank is a serious health hazard for your fish. They’ll be swimming in their own toxic waste as they’ll produce more waste than you can remove, and oxygen levels in water will drop.

Plus, overstocked aquariums are stressful, it can lead to aggressive behaviors, which in turn can cause injuries.

High toxicity and low oxygen levels will lead to all sorts of molly fish diseases and will ultimately kill off your fish.

Therefore, know how many fish you can add to the tank based on size and volume, so you don’t end up cramming in more fish than it can comfortably accommodate.

– Adding live plants to the tank reduces the number of mollies you can keep

Live aquatic plants are an excellent way to recreate the natural habitat of molly fish, to offer them hiding spaces and shade, and help keep the water clean and clear.

Plus, some mollies enjoy nibbling on plants or algae that grows on these plants. But plants also take up space in your aquarium, which means you’ll have to cut down the number of mollies you want to keep.

Therefore, if you have a 20-gallon planted aquarium, it can comfortably accommodate up to 4 mollies, while a 30-gallon planted aquarium will accommodate up to 6 mollies.

How to Stock Molly Fish?

To avoid overcrowding in the tank and to prevent molly population from getting out of hand, there are a few rules to follow when stocking your molly fish aquarium.

Since livebearer fish breed so often, the male will constantly be in pursuit of the female molly fish. Now, if there are too many males, female fish will get stressed out by the constant pestering from male fish.

Therefore, keep one male for 2-3 females to avoid stressing females out and to avoid the production of a lot of fry.

Although adult molly fish eat their own fry, the fry that end up surviving in an adult molly fish tank may be few, if your fish breed constantly, things can and do get out of hand pretty fast.

You can also opt to keep only females or only males, however, this may cause its own set of problems – bullying, injuries, fights, etc.

Do Molly Fish Need a Filter?

An aquarium filter is a crucial piece of equipment in any fish tank. Filters serve many purposes including:

– They keep the water clean by removing decaying substances;

Decaying substances release toxins into the water, overthrowing the carefully constructed balance during the nitrogen cycle.

Fish end up swimming in their own waste, which is extremely harmful for them.

– They remove debris and uneaten food particles;

Physical debris along with uneaten food particles cloud the water and are also a source of toxins. Toxins can also promote the growth of pathogenic bacteria and parasites, which will attack your fish, endangering their lives.

– They rid the water of waste products and chemicals;

Filters clean the water not only of waste products but also of harmful chemicals. Chemical filter media remove any trace amounts of chlorine and heavy metals that may linger in water after it was conditioned.

If you use tap water for water changes, it’s crucial that you treat the water before adding it to the aquarium. Use a water conditioner like SeaChem that neutralizes chlorine and other chemicals.

Still, it doesn’t hurt to add an extra layer of protection in the form of a filter that will further absorb any trace amounts of harmful chemicals that may be left behind.

– The filter media in an aquarium filter is an excellent site for good bacterial colonies to form

The filter is an excellent site to be populated by healthy bacteria that transform toxins into less dangerous substances.

This is the reason why all aquarium maintenance guides warn you not to wash your filters with chlorinated water, so you don’t kill off the healthy colony that has formed on it.

Simply rinse the filters in aquarium water and replace the filter media only when it reaches the end of its service life.

With so many benefits, aquarium water filters are an indispensable device for any fish tank, including molly fish that are known to produce a lot of waste.

Depending on how stocked your aquarium is, live plants coupled with regular water changes may not be able to remove waste at the same rate as it’s being produced by your fish.

With a quality filter, you make sure waste removal is performed at rate that’s beneficial for your fish. Therefore, a filter is highly recommended for a molly fish tank.

Since mollies are smaller, be careful which filter you get to prevent them from getting sucked into the system. Alternatively, you can place a mesh on the inlet to prevent accidents of this sort.

Also, noise levels are also important. Some fish can get stressed out by a filter that’s too noisy.

Moreover, a filter that’s too powerful can hinder the swimming ability of some mollies. Balloon Molly fish and Potbelly Molly fish have a more rounded shape that makes them less competent swimmers compared to other mollies.

Do Molly Fish Need an Air Pump?

Aquarium air pumps agitate the surface of the water in the tank in order to increase the level of oxygen exchange that occurs in the aquarium.

Like all living creatures, mollies too need oxygen to survive, and the lack of enough oxygen can bring about a series of health issues including death.

Air pumps operate by creating bubbles in the water that then rise to the surface “agitating” it. This drives more oxygen into the tank, creating a healthy environment for them.

Besides the role of being a “surface agitator”, an aquarium air pump can be fashioned as a secondary filter if the end of the pump is fitted with a sponge filter. It will help clear the water and maintain quality parameters.

Clearly, an air pump is beneficial for an aquarium, but do mollies need an air pump? Or will they do just fine without it?

An air pump or air stone is great at facilitating oxygen exchange in the water, but there are other means to achieve that (e.g. adding live plants to the tank), so an air pump is not an absolute requirement for a molly fish tank.

Do Molly Fish Need a Heater?

If you can get away without using an air pump in your aquarium, a heater is indispensable for mollies.

Aquarium heaters increase the water temperature in the tank and maintain it at the level required by your fish.

This way, sudden drops in temperature are avoided. Unfavorable temperatures and acute changes affect the immune system of your molly fish, opening up the way for diseases and illnesses.

Since mollies are tropical fish that need warmer water that you may not be able to create simply by heating your apartment, a heater is required for molly fish tanks.

If you live in a tropical climate with warm weather all year around, a heater is not required. But if you live in a region with below freezing temperatures, there’s no way to maintain the optimal temperature level in the aquarium; you need a heater.

Water temperature for mollies should be between 75 F and 82 F, therefore, choose a heater that can help you maintain a stable temperature in this range.

When sizing the heater, you should follow the 5 watts per gallon rule. Therefore, if you’re setting up a 30-gallon molly fish aquarium, choose a 150 watts heater.

It’s important to size the heater accordingly. A low-wattage heater will not be able to heat up the water in the aquarium to the required temperature, and you’ll just end up having to switch to a bigger heater anyway.

Choose a heater made of durable materials. Plastic is not a good option as heaters made of cheap materials can break or malfunction.

Pick a heater with a thermostat, which will keep the temperature at the level you’ve set it to, regardless of the changes of air temperature in your room.

What About Substrate? What Type of Substrate is Suitable for Mollies?

Mollies aren’t picky about their substrate. You can go with pebbles/rocks, gravel, or sand, they’ll like it either way.

But since they produce a lot of waste, gravel may be a better choice since it’s much easier to clean than sand, for example.

If you plan on keeping live plants in the aquarium, you will need to consider a substrate that’s able to sustain plant life as well. All-in-one aquarium substrates are the best choice in this scenario.

Choose plants that will do fine in low light conditions as well, so you won’t need to set up a lighting system for the aquarium.


Molly fish are seemingly undemanding and easy-going fish, however, they do have specific requirements that must be met to ensure they stay healthy throughout their lives.

If you are aware of these requirements and you manage to meet them, then your experience with molly fish-keeping will be a rewarding and fulfilling one.

Many beginner aquarists make the mistake of taking the description that mollies are hardy fish too literally.

Yes, mollies are hardy compared to many other fish that are much more sensitive, but they do have their own requirements that must be met.

Make sure to offer you molly fish plenty of space for swimming and hiding, if possible, add live plants to the tank, don’t overlook the importance of filtration and heating, and don’t overcrowd the tank.

Please remember that the aspects I touched upon in this article, refer to molly fish in general, and particularities may be different from breed to breed, therefore, make sure you do your research on the breed you end up choosing for your aquarium.

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Questions and Answers

I only have a 17 litre tank which pets at home sold me , telling me it would be ok for 6 mollies. I now only have 3 left.the one’s i have lost start off rolling & spinning in the tank . I treat them it then stops it for two weeks then starts doing it again, then dies . Please can you help as this was my 12yr daughters birthday present.salann.

    Most likely the tank was not cycled and your fish died due to ammonia poisoning. You need to cycle the tank first. This means, that you setup the tank without fish, for at least one week, add beneficial bacteria and run the filter. After one week, you can add 1-2 fish, so they start producing ammonia. The ammonia will be converted to nitrites and then to nitrates by the beneficial bacteria from the filter.
    After another week, you can increase the number of fish. A tank is considered cycled after 6 weeks.
    So, I suggest letting your tank to cycle. Do weekly water changes and hope that your remaining fish will not die. After the 6 weeks passes, you can add more fish. In a 17 liter tank, you can add 3-4 mollies. I would not add more than that, because mollies do grow to a decent size and the tank will be small for them.
    Do not overfeed the fish. Feed once a day a small pinch of fish food. That will be enough for your fish.

TigerQUEEN2012 May 4, 2023 Reply

Can I keep Mollies and Guppies togever?I was thinking both female just because I am not intrested in breeding.How many Mollies,how many Guppies?

    Sure, you can keep guppies and mollies in the same tank. You can add 3 guppies and 3 mollies, just make sure you have enough space for them.

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