Molly Fish Care: Expert Guide & Tank Set Up For Beginners

Mollies (Poecilia sphenops) are wonderful fish, and come in a huge variety of colors and fin shapes.

Hardy and active, if given the right conditions, they’ll live for 3-4 years and produce thousands of babies in their lifetime.

And in this guide, I’ll teach you everything you need to know about caring for them.

Scientific Classification

• Kingdom: Animalia
• Phylum: Chordata
• Class: Actinopterygii
• Order: Cyprinodontiformes
• Family: Poeciliidae
• Genus: Poecilia
• Species: P. sphenops
• Binomial name: Poecilia sphenops

Overview: In the Wild

There are several different species of fish sold under the name “molly.” They all come from the genus Poecilia and their native range runs from Central America, up along the Gulf and Atlantic coasts, ending in the Carolinas.

Mollies are tough little fish that can adapt to a wide range of conditions in the wild.

They can be found in fresh, brackish or even saltwater environments.

Molly fish typically inhabit areas with fairly dense vegetation that they use as cover from predators. They shoal together and search for algae and small crustaceans.

Varieties

Through selective breeding and hybridization, many different varieties of mollies have been developed.

They fall under two main groups:

• Shortfin mollies – overall body shape looks similar to platy fish, but with a deeper chest and belly. They have smaller tail and fins that really aren’t very showy. These varieties are usually smaller, only growing to 4 inches (10 centimeters) or so. Color patterns include:
  • Common Black
  • Gold Dust
  • Gold Doubloon
  • Red Sunset
  • Dalmatian 
• Sailfin mollies – much larger dorsal and caudal (tail) fins. They are also often larger than shortfins, some getting up to 6 inches (15 centimeters) long. Varieties include:
  • Creamsicle Sailfin
  • Golden Sailfin
  • Dalmatian Lyretail
  • Platinum Lyretail
  • Harlequin Sailfin

These are just a few! You’ll see even more available in live fish stores.

Balloon Mollies 

I did want to note that you’ll often see a variety known as the balloon molly available for sale.

Balloon mollies are extremely controversial because their big bellies are the result of a genetic mutation that curves their spines which compresses their internal organs, forcing their abdomens to bulge.

Many see this practice as painful and cruel. They argue that these fish should not be bred or sold.

Others will argue that they’ve seen plenty of healthy balloon mollies happily swimming around tanks.

I can’t give definitive proof that the fish are always in pain, as some claim. But I can say that the majority of aquarists report that balloon mollies don’t live as long as other varieties. 

Their compressed abdomens do make them more susceptible to constipation, extreme cases can be fatal. 

I would lean towards not buying balloon varieties. Even if the fish aren’t in constant pain, the shorter lifespan would rule them out for me.

Imported Mollies

Most mollies are bred in Asia and then shipped all over the world. Especially those sold in the big chain stores.

The breeders typically rear them in salt or brackish water. Freshwater costs money, but they can access brackish/saltwater for free.

Mollies can easily thrive in the high salinity, but they can have major issues when they get shipped to stores and are suddenly plunged into tanks with no salt in the water at all.

Over time, their kidneys can shut down because their bodies are used to living in saltwater.

Pro Tip: If at all possible, get your mollies from a breeder that has reared them in freshwater.

Setting up a Molly Tank

Water Parameters

• Temperature: 75°-80°F (24°-26.7°C)
• Ammonia/Nitrite: 0
• Nitrate: <30 ppm
• pH: 7.5-8.5
• GH: 12-25 dGH (200-416 ppm)
• KH: 10-25 dkh (178-450 ppm) 

Mollies originate from waterways with a high mineral content and pH levels. 

I’ve seen lots of tank bred fish adapt to a wide variety of water conditions, hard water fish living in soft water and vice versa.

But mollies are a bit of an exception. They really can’t thrive in soft water. If you live in an area with soft water, you will need to add minerals to your water.

Pro Tip: Adding crushed coral to your substrate, or in a mesh bag in your filter, can help increase the hardness and pH of your water. The crushed coral will slowly leach calcium and carbonate into the water column.

Salt or No Salt?

There’s a huge debate among aquarists about whether or not you should add salt to the water in a molly tank.

I actually have to say that both sides of the argument are right. 

Mollies don’t actually need salt in their water. They can just adapt to saltwater/brackish conditions if they have to.

But, adult mollies that have been reared in nothing but saltwater may need salt added to their tank because their bodies are used to living with high salinity. 

Pro Tip: When buying mollies, ask whether or not they were imported so you can decide about adding salt to your tank. You can assume that mollies from big chain stores were bred overseas and will need some salt in the water, at least at first.

Equipment

Heater 

Mollies are a tropical fish that needs temperatures in the range of 75°-80°F (24°-26.7°C).

A heater is an absolute necessity that will keep the water warm and hold it at a steady temperature.

Filtration 

Mollies might be super cute, but they’re also really messy. Basically, they’re little machines that do nothing but eat, poop and make babies.

They constantly gobble food like they’re starving and so they produce a lot of poop. 

All that waste sinks to the bottom of your tank where it starts to break down, putting off ammonia (NH₃). This is a bad thing since even a small amount of ammonia is toxic to fish and invertebrates.

Lucky for us fishkeepers, there are beneficial bacteria that live in our filters. One kind eats ammonia and turns it into nitrite (NO₂ -1), another toxic byproduct. But then another species of bacteria quickly eats the nitrite and puts off something called nitrate (NO₃-).

This whole process is known as the aquarium nitrogen cycle.

Nitrate is much less toxic and can be allowed to build up in the aquarium in between water changes.

Pro Tip: Getting the nitrogen cycle up and running in a new tank doesn’t happen instantly. For more information on how to get your tank ready for fish, check out our steps for performing a fishless cycle.

But you need to make sure that you have a filter that can hold enough beneficial bacteria to keep up with processing the tons of waste put off by mollies. Get a filter that can hold a lot of biomedia and always add as much filtration as possible, maybe even adding on an extra filter, like an internal or sponge filter. 

Substrate 

Mollies will benefit from an aragonite sand substrate. Aragonite is made of calcium carbonate (CaCO₃) that will slowly leach minerals that the fish will appreciate.

If you’re absolutely set on a gravel substrate, consider adding several cups of crushed coral to the gravel. It will function the same as aragonite.

Plants and Decor

Mollies are an active, top dwelling fish. So they need lots of open swimming areas.

But, they also enjoy densely planted areas and maybe some rocks or decor that provides a few hiding areas. Sometimes you just need a little privacy from your family, right? 

Tank Mates

It’s important to note that mollies are extremely social shoaling fish. So, they’re happier if kept in groups.

It’s best to only have one male for every three females. Males harass females constantly, trying to mate with them. Having several females means no one fish is being stressed by the male’s attention.

For tank mates from other species, mollies are fairly peaceful fish, but they do get a lot bigger than many species. They’re also greedy feeders that can outcompete smaller fish for food.

Because of this, I’d recommend going with other peaceful but fast-moving community fish, or fish that inhabit a different level of the aquarium. 

Some good choices are:

• Corydoras catfish
• Swordtails
• Clown and Bristlenose Plecos
• Platies

Diet 

Mollies really are not picky eaters and will gladly devour any food you put in front of them. 

A high quality flake food, like ZooMed Spirulina 20 or Omega One Freshwater Flakes makes a good staple diet.

Last update on 2023-05-30 / Commissions Earned / Images from Amazon Product Advertising API

Mollies also really enjoy picking at algae. So, it’s good to provide them with a varied diet that includes algae in the ingredients, like Omega One Veggie Rounds or Repashy Soilent Green.

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Giving them a healthy, varied diet is important for overall fish health and improves their color.

Breeding Molly Fish

Mollies are livebearers, meaning they give birth to live fry instead of laying eggs. 

Breeding mollies is really easy:

1. Fill a tank with water.
2. Put mollies in it.

It’s pretty much just that easy.

But seriously, healthy mollies will produce batches of fry every month or so. 

Adults will eat the fry so they need lots of hiding areas, like in dense areas of live plants, so they can avoid hungry adults until they grow too big to eat.  

Are Molly Fish Right for You?

Mollies are really interesting fish that come in a huge variety of colors and different fin shapes.

They’re a hardy fish that’s always active. Given the right conditions, they can live for 3-4 years and produce thousands of babies in their lifetime.

So it’s important to decide whether you’re just going to let adults eat the fry, separate the fry and raise them and what you’d do for homes for the fry that make it.

It’s best to have a plan in place so you don’t end up with a bunch of baby mollies that you don’t know what to do with.

It’s also important to consider that mollies get quite a bit bigger than many community fish. You’ll need a larger tank to keep this species since they put off so much waste.

Overall, I do think they make a great pet, if you have the right conditions to house them. They have inquisitive personalities that you’ll find quite charming.