How Many Mollies In A 10-Gallon Tank? It’s Not Really Recommended!

Dec 26, 2021

How Many Mollies In A 10-Gallon Tank?

Credits: Marrabbio2 (Creative Commons license)

A 10-gallon tank is often marketed as a starter, but that’s nothing more than a marketing gimmick. Smaller tanks are a lot more challenging to maintain than big ones. And while mollies aren’t exactly high-maintenance fish, they do need a sizable home. So, how many mollies in a 10-gallon tank would be ideal? In fact, is it even possible?

You will find out soon! 

How Many Mollies In A 10-Gallon Tank? 

I wouldn’t recommend adding mollies to a 10-gallon tank. These fish make active swimmers and produce a good amount of bioload every day. However, if you go by the ‘one inch’ per gallon rule, you can house 3 3-inch long mollies and 2 4-inch long mollies in a 10-gallon tank. 

But note that mollies are shoaling fish that need to be kept at least in a group of 6 or more to thrive and feel comfortable. And a 10-gallon tank doesn’t provide space for that. 

Also, if you’re planning to build a community tank, don’t forget to take other inhabitants’ size and spatial needs into account. 

The ‘one inch per gallon’ rule is controversial for several reasons. First, it doesn’t consider the space taken by decors and substrate during calculation. Next, it completely ignores the fish’s height. 

However, the consensus is that this rule can be applied to slender-bodied fish smaller than 3-4 inches. 

How Many Male And Female Mollies In A 10-Gallon Tank? 

If you are a beginner, the starting place for developing a colony of livebearing fish is 1 male and 2 females. The ideal environment would have bushy, floating plants. 

Having two females for one male will help spread out the attention and subsequent harassment the females will receive. They will at least get some rest in between. 

Lastly, the bushy, floating plants will offer places to hide for molly fry. I’d recommend adding hornwort. It will easily house and foster microorganisms that your fry will readily snack on. Plus, it helps maintain the water quality by ingesting ammonia. 

How Many Black Mollies In A 10-Gallon Tank?

A black molly
Credits: Pmalkowski (Creative Commons license)

Blackfin mollies can grow anywhere between 3 to 6 inches. The females are usually a couple of inches bigger than males. If your black mollies max out at around 3 inches, you could add about 3 of them in a 10-gallon tank. 

However, if the fish grows to be 6 inches long, even adding 2 of them in a 10-gallon tank would be a stretch. 

How Many Sailfin Mollies In A 10-Gallon Tank?

Sailfin molly
Credits: Falashad (Creative Commons license)

Sailfin mollies seldom grow over 5 inches long. So, according to the calculation, you can add 2 sailfin mollies in a 10-gallon tank

But you’d only get away by the skin of your teeth if you add 2 5-inch fish in a 10-gallon tank. 

How Many Dalmatian Mollies In A 10-Gallon Tank?

Dalmatian molly

Dalmatian mollies grow around 4.75 inches long in captivity. So, you could house 2 of them in a 10-gallon tank. 

However, this breed is known to be social and active. They love to school. So, keeping 2 of them in a 2-gallon tank is definitely not recommended. 

How Many Lyretail Mollies In A 10-Gallon Tank?

Credits: Segrest Farms (Creative Commons license)

Female lyretail mollies can grow up to 5 inches long. Thus, you can only house 2 of them in a 10-gallon tank. 

But males are a bit shorter – reaching only 3 inches. So, you can technically have 3 of them in a 10-gallon tank. 

However, adding only males in a tank that small is a bad idea – they will literally fight to the death. 

How Many Gold Doubloon Mollies In A 10-Gallon Tank?

Gold doubloon mollies grow around 5 inches long. So, once again, going by the ‘one inch per gallon’ rule, you can add 2 of these fish in a 10-gallon tank. 

As a matter of fact, these fish at least need a 25-gallon tank with plenty of room to swim in and algae to feed on. 

Recommended Readings!

How To Tell If A Molly Fish Is Dying? Look For These Signs!

How To Care For Pregnant Black Molly Fish?

Everything You Need To Know About Balloon Belly Molly

How Long Do Mollies Live? Extend Their Lifespan!

How Long Are Molly Fish Pregnant For? Twice As Long As Guppies!

What Do Molly Fish Eggs Look Like? Little Yellow Balls!

How Many Mollies Should Be Kept Together? 

Mollies are shoaling fish. They love to stick together. Therefore, the absolute minimum number of mollies you can keep is 4. And for that, you would at least need a 20-gallon tank or more. 

If you plan to manage multiple shoals of mollies, I’d recommend getting a 45-gallon tank. 

I have added a section in my blogs where I pool in the relevant comments left by hobbyists of different forums. This is my initiative to give you well-rounded and neutral information to help you make informed decisions. 

So, what do people have to say about keeping mollies in a 10-gallon tank?

Let’s find out! 

How Many Mollies In A 10-Gallon Tank? Real Answers By Real People! 

The general rule is “1 inch of slim-bodied fish per gallon”. So, how many mollies could you keep safely in a 10-gallon tank? I’d say no more than 5 – preferably 4. Good luck!”

“I would suggest 3-4 mollies of the same sex. Opposite sexes will have your tank overrun very fast. You’re talking about black (includes marbled, gold dust, and dalmatian) mollies, right?”

“It really boils down to the molly type and the salt content of the water in my experience. If you buy a half dozen tiny black mollies and raise them in freshwater, they won’t grow huge. If you purchase some green sailfins and keep them in a brackish environment, they’ll get huge – way too huge for a 10-gallon tank.”

“I personally would never keep 4 in a 10-gallon tank at all. I suspect this would stunt the fish’s growth, which is really not kind fishkeeping.”

“10 gallons is just too small for this type of fish. 20 gallons is the smallest tank size I’d consider for mollies.”

“Mollies tend to mature between 6 to 8 months. So, a 10-gallon tank is quickly going to be small. While some folks out there say 10 gallons is okay, a 20-gallon tank is far more suited.”

“Personally, I think a 10-gallon tank should be left for bettas and small schooling fish. When I started out, I kept 2 mollies in a 10-gallon tank. One ended up killing the other. Now, I wouldn’t keep them in anything smaller than 55.”

As you can see, the comments are really a mixed bag. While some think mollies in a 10-gallon tank is doable, others strongly advise to get a bigger tank.” 

“The thing with mollies is that they have higher activity levels, poop constantly, and actually grow to a decent size. So, a larger tank would help maintain water conditions and keep diseases in check.

“This is especially true for the balloon varieties that are more susceptible to diseases due to the formation of their stomachs.”

“Don’t get disheartened, but I’ll now tell you a few reasons why mollies aren’t suited for a small tank.

4 Reasons You Shouldn’t Keep Mollies In A 10-Gallon Tank 

  • Your mollies will get stressed 
  • Your mollies will get lonely 
  • A small tank can stunt growth
  • Small tanks are difficult to maintain 

Your Mollies Will Get Stressed 

A small tank comes with a host of disadvantages. To name a few, the fish won’t have enough space to swim and explore, there will be territorial disputes, subdominant fish will get bullied, and the tank will get polluted in no time.

All of these factors, alone or combined, are more than capable of causing your fish unsolicited stress. When stressed, the fish’s hormones fluctuate. One such hormone is cortisol – a stress hormone. 

The fish can simply swim away from the stress-inducing factor in the wild. The hormones will then reset to normal. 

However, in the tank, the fish will stress until you do something about it. 

So, by the time you figure out the fish is stressed and take necessary steps, stress will profoundly impact the fish’s wellbeing. 

For example, a fish’s protective mucus layer gets frail when stressed. As a result, the fish will become vulnerable to pathogens waiting for the right moment to strike. 

Your Mollies Will Get Lonely 

Mollies are schooling fish. So, it’s not wise to deprive them of this natural instinct. A 10-gallon tank can barely house 2 mollies – so, there’s no way they will school. 

Experienced hobbyists suggest the minimum number of mollies you should keep is 4. And if you really want to see their unique schooling behavior, you should get 6 or more. 

Adding only 2 mollies to a small tank will deprive them of the socialization they thrive on – making them vulnerable to stress and impoverished life in general. 

A Small Tank Can Stunt Growth 

Research has shown that fish release a certain pheromone into the water that can negatively impact the growth of their own species. These studies mainly focus on fish like goldfish that belong to the carp family. 

But we still can’t rule out this possibility for mollies as well. 

In the wild, this isn’t really an issue as these chemicals will get diluted or washed away instantly. However, it would raise eyebrows in a closed system like an aquarium, wouldn’t it?

The smaller the tank, the higher the concentration of these growth-inhibiting compounds will be. So, it’s safe to assume that small tanks prematurely stunt fish growth. 

Also, mollies are active fish that love to swim around all day. A small tank wouldn’t make a great playground. They won’t get the exercise they need.

And it wouldn’t be farfetched to link lack of exercise with stunted growth, would it?

Small Tanks Are Difficult To Maintain

Contrary to popular belief, large aquariums are much easier to upkeep than smaller ones. 

To give you an example, a small amount of leftover food in a small tank would produce a higher ammonia concentration than the same amount in a larger tank. In a sense, a large aquarium offers more room for mistakes. 

In small tanks, even the smallest mistakes are magnified. Even then, the most minor fluctuation in the environment can lead to grave consequences. 

And what upsets me is the fact that a 10-gallon tank is often advertised to be perfect for a beginner. No, go bigger – you won’t regret it. 

Mollies grow to a decent size. And mind you, produce a decent amount of bioload every day. A small tank would keep you on your toes all day! It will be a hassle. 

So, we recommend at least a 20-gallon tank for mollies. 

Frequently Asked Questions 

Credits: h080 (Creative Commons license)

What’s The Minimum Recommended Tank Size For Mollies? 

Although the minimum recommended tank size for mollies is 10-gallon, a tank that small can only house 2 mollies. And it’s cruel to keep only 2 mollies. 

We recommend at least a 20-gallon tank for four mollies. If you like to keep multiple shoals, upgrade to a 45-gallon tank. 

What’s The Ideal Water Parameters For Mollies Like?

Here’s what water parameters for mollies look like:

Temperature75°-80°F (24°-26.7°C)
pH7.5-8.5
GH200-416 ppm (12-25 dGH)
KH178-450 ppm (10-25 dGH)
Ammonia/Nitrite0 ppm
Nitrate<20 ppm

How Many Mollies In A 5-Gallon Tank?

You can keep only 1 molly in a 5-gallon tank. But if you have a 5-gallon tank, stick to other species like betta, shrimps, and snails. 

A molly isn’t suited for a 5-gallon tank. It’s too small for them. 

How Many Mollies In A 20-Gallon Tank?

You can keep around 4 mollies in a 20-gallon tank. Keep them in a ratio of 3 females for a single male. 

This helps to spread out the aggression and harassment the male will inevitably lash out on females. 

How Many Mollies In A 36-Gallon Tank?

A 36-gallon tank can hold around 8 mollies. That’s a perfect number if you want to witness their schooling behavior. 

Once again, note that females should outnumber males as the latter tend to be hostile and temperamental. 

How Many Mollies In A 50-Gallon Tank?

Assuming the mollies’ average length to be 4 inches, you can house 12-13 of them in a 50-gallon tank according to the ‘one inch per gallon’ formula. 

But from what hobbyists shared on forums, you can safely keep 20 (maybe 25?) mollies in a 50-gallon tank, provided that it has excellent filtration. 

However, I’d still recommend understocking as it makes the fish easier to look after. 

How Many Mollies In A 75-Gallon Tank?

The “one inch per gallon” rule suggests that you can keep about 16 mollies in a 75-gallon tank. 

Keep them in a ratio of 3 females for every 1 male.

How Many Mollies And Guppies In A 10-Gallon Tank?

A 10-gallon tank will barely accommodate 2 mollies. So, there’s no chance you can add guppies to that tank. Sorry! 

Final Words: How Many Mollies In A 10-Gallon Tank?

Adding mollies in a 10-gallon tank is not recommended. The tank can technically house a few of them (2-3), but the fish will undoubtedly lead an impoverished life. 

For starters, mollies are active and inquisitive fish. A 10-gallon tank would neither provide enough room for swimming nor entertainment to keep the mollies occupied. 

On top of that, mollies are shoaling fish. Ideally, you should at least keep them in a group of 6 or more for them to thrive and feel happy. And as you can guess, a 10-gallon room can’t accommodate half a dozen mollies. 

Lastly, although 10-gallon tanks are often advertised to be easy to maintain and beginner-friendly, the opposite is true. They’re more temperamental, prone to pollution, and offer no room for mistakes. Go for a bigger tank – it may sound intimidating – but it will keep both you and your mollies happy! 

Recommended Readings!

How Many Neon Tetras In A 20-Gallon Tank? 10 or 30?

How Many Tetras In A 30-Gallon Tank? One Inch Per Gallon?

How Many Neon Tetras In A 5-Gallon Tank? Honest Answers!

rohit gurung author at urbanfishkeeping

About Rohit Gurung

My never-ending love and fascination with Aquascaping started when I received a red-eared turtle for my 10th birthday.

Apart from researching and writing, I spend hours gazing at my 3 turtles. And yeah, I bask alongside them too.