Molly Fish vs Swordtail Fish – What is the Difference?

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If you’re setting up a freshwater tank with tropical conditions, you’re a lucky one. Tropical freshwater aquariums have the most options in terms of fish species available.

Today, we will discuss 2 of the most popular fish species you can get for your freshwater tank: mollies and swordtails.

Molly vs Swordtail – Similarities and Differences

While both these fish species share a lot of similarities, they are also different in many aspects. Let’s see how you can distinguish the 2 based on several factors.


What are the main differences in appearance between molly fish and swordtail fish?


Mollies are easy to identify since they have standard bodies for a livebearer. They’re called short-finned mollies for a reason. The fish is stocky with a compact body and a thick belly, which grows even larger in females. The fins are short, and so is the tail.

In essence, mollies are guppies but without the fluffy fins and maybe not as flashy. Mollies can grow up to 5.5 inches, which is more than double what a guppy can reach.

Fortunately, mollies display quite an impressive range of colors and patterns, making up for their rather bland physical appearance.

Some notable entries here include:

  • Black molly – Everything is black, including the body, fins, and even the eyes. Only the tail showcases some subtle yellow on the margins. Black mollies are great in colorful setups, creating a powerful contrast with other, more color-rich fish.
  • Dalmatian molly – A white body covered with random black spots and splashes. Some even come with hints of yellow in the mix.
  • Balloon belly molly – This is among the few mollies with an uncommon body shape. While balloon mollies come in different colors, their body shape stays the same. This molly has a hunchback and a protruding belly, forcing its head up a bit. This one is the result of selective breeding, which is why you won’t find it in the wild.
  • Gold dust molly – Picture a yellow fish doused in a mix of gold dust and ash. That’s exactly what this one looks like.
  • White/Black sailfin molly – Sailfin mollies have large dorsal fins that they open up when swimming. This makes the fish resemble a boat, hence, the name.

In short, there is a lot of diversity here, especially in terms of coloring and patterns. Mollies are sure to diversify your tank considerably, especially when mixing different types.


Swordtails grow slightly larger than mollies, up to 6.5 inches. They will generally remain smaller than that, though. In terms of body structure, swordtails are elongated and aquadynamic with a wider dorsal fin.

But the thing that stands out the most is the fish’s caudal fin. Swordtails have the tail’s lower lobe long and sharp, similar to a sword, that is often as long as the fish’s entire body.

Only males exhibit this extended caudal fin, though, which makes sexing the fish easier.

Just as with mollies, swordtails also exhibit quite the diversity in terms of coloring, color patterns, and even body structure.

In this sense, we have several swordtail types to consider, such as:

  • Yellow swordtail – The fish is stocky and small, as it only grows up to 2 inches. The caudal fin is also much shorter than the body’s length and yellow with black margins. The fish itself is a mix of grey and yellow with 2 longitudinal orange stripes traversing the body from head to tail.
  • Chiapas swordtail – A long (up to 6 inches) swordtail with a light orange body and a long and sharp caudal fin. The fish is incredibly slim and athletic compared to bulkier types.
  • Sheepshead swordtail – This one wanders away from the general bunch. Sheepshead swordtails lack the trademark caudal fin but make up for it in the dorsal department. Males exhibit large and wide dorsal fins meant to intimidate other males and attract the attention of females.
  • Montezuma swordtail – I’ve said the best one for last because the Montezuma swordtail is unlike any other swordtail you may have seen. This is an emerald-colored fish displaying a uniform, glassy green, and an obscenely long caudal fin. The caudal lobe is inclined downward and is sometimes longer than the fish’s whole body.

As you can see, swordtails also display astounding variety, making them great additions to any community tank.


What are the water and tank size requirements for mollies and swordtail fish?


Mollies are peaceful fish that like to live in active and energetic communities. So, you’ll need to have several of them for good measure.

The company of their own makes the fish feel safer and calmer.

A minimum of 10 gallons is necessary to accommodate several smaller mollies, making this fish great for nano setups. This mid-dweller requires temperatures around 75-80 °F, although some mild variation is acceptable.

The problem with mollies is that they demand a clean and healthy habitat because these ranks as the most sensitive livebearers available.

Mollies are more prone to infections and parasites in unclean environments.


Swordtails are adaptable and playful, and they like to swim a lot. So, provide them with sufficient swimming space, preferably in the 15-20-gallon range.

You may want to consider at least 30 gallons for larger groups and community setups.

The ideal temperature sits between 70 and 80 °F, as is normal with tropical fish.

Fortunately, the swordtail isn’t as sensitive as mollies are, but it still requires clean and healthy waters.

A clean and fresh aquatic environment will boost the fish’s lifespan and keep it healthy, active, and more colorful over the years.

Food and Diet

What do mollies and swordtail fish eat? How should you feed these fish?


Mollies rank as omnivorous fish, but their diet leans more on the veggie side. These are natural plant and algae eaters in the wild, and they display a similar behavior in captivity.

So, make sure that your mollies have a lot of spirulina, algae, and veggies at their disposal daily with occasional meat-based treats 2-3 times per week.

Some good protein sources include brine shrimp, bloodworms, daphnia, mosquito larvae, etc.


Swordtails are also omnivorous fish, but they don’t need as many veggies. Their diet is a bit more balanced, as they require slightly more animal protein than mollies.

Daphnia, brine shrimp, white fish meat, and bloodworms are all good options in this sense.

Supplement the swordtail’s diet with some spirulina, spinach, and various veggies here and there, and they will thrive.

When it comes to feeding the fish properly, go for 2 meals per day at most and keep them short.

Only provide the fish with the food that they can eat in 2 minutes tops. Everything above that will result in more food leftovers decaying on the substrate and increasing ammonia levels.


How do mollies and swordtails breed, how many fry they have and how often do they give birth?


Molly generally breeds every 60 days because that’s the length of their gestation period. This being said, females can store the males’ sperm in a special belly pouch for several months after mating.

This allows the female to fertilize its own eggs every month and produce fry every 60 days without having any contact with any males.

This is also typical behavior in guppies.

You can tell that your molly female is gravid by assessing its belly size and looking for the distinct gravid spot.

The gravid spot is located near the tail, in the abdomen’s lower region, and it grows larger and darker as the delivery time approaches.


Swordtails are also livebearers and prolific breeders, capable of producing fry monthly. As is the case with most livebearers, swordtails display no parental instinct and will eat their own fry after hatching.

So, it’s best to separate the gravid female from the general pack until the fry are born. You should also apply this strategy to mollies for the same reasons. A nursing tank is ideal in this case, allowing the fry to grow uninhibited without fearing that larger fish will eat them.

The fry should be good to relocate to the main tank once they’re 1-month old. You can increase that period to 2 months just to be sure.


Both mollies and swordtails live up to 5 years in captivity. Their lifespan is in close relation to their quality of life.

In this sense, consider the following:

  • Provide the fish with a diverse, nutritious, and steady meal plan
  • Vacuum the substrate and clean the habitat regularly
  • Have a reliable filtration system to keep the water clean and well-oxygenated
  • Don’t overstock the fish to prevent stress, territorial fights, and bullying
  • Don’t pair your mollies and swordtails with aggressive or territorial species for the same reasons
  • Keep them in a natural-looking setting with substrate, live plants, rocks, and other aquatic decorations
  • Monitor your fish’s dynamics to prevent hierarchical violence, food competitions, or mating-related aggression
  • Monitor the fish constantly to detect health problems in time and increase their chances of recovery

Molly or Swordtail Fish are Best for Community Tank?

They are both great for community setups, as they are friendly, docile, and generally peaceful. That being said, you should choose their tankmates carefully.

In this sense, consider the following tips:

  • Keep them in groupsMollies and swordtails feel safer and calmer when in the company of their own. They’re not schooling fish but like to live in tight communities with others of their own species. Have at least 4-5 mollies and an equal number of swordtails, no matter the profile of the community setup.
  • Have sufficient hiding areas – Both mollies and swordtails are non-combative, so they will avoid conflicts as much as possible. In this sense, having a variety of hiding areas will keep the fish calm and safe. Otherwise, they can get stressed out, and stressed fish are always more prone to sickness and parasites.
  • Find compatible tankmates – Avoid large, aggressive, or overly curious species. Guppies, tetras, tiger barbs, angelfish, and dwarf cichlids are all viable options in this sense.

How to Tell the Difference Between Molly and Swordtail Fry?

You may not be able to distinguish the fry at first since they’re all the same. The differences will become more visible as they grow, though, as molly fry will get rounder and showcase smaller fins.

Eventually, swordtail males will develop their trademark caudal fin, but this happens a bit later on, past the 1-month mark.

Can Mollies Mate with Swordtails?

No, mollies and swordtails cannot procreate with each other. That’s primarily because they belong to different families. Mollies are Poeciliiae, while swordtails are Xiphophorus, making them genetically incompatible.

This being said, you will come across people claiming that their mollies and swordtails have been able to procreate.

This is nothing more than a case of confusion due to the mollies’ ability to store the male’s sperm.

So, we’re talking about cases where people have several molly females, and one or more swordtail males and mollies are still able to produce fry.

That happens because the molly uses the sperm it stored from previous encounters with molly males. It doesn’t use the swordtail’s sperm since it’s incompatible.

The more you know, right?


Mollies and swordtails are friendly, colorful, diverse, and easy-to-breed fish that will complement any aquatic setup.

Provide them with good food, a steady meal plan, and a clean, natural-looking habitat, and they will be with you for years to come.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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