Freshwater Fish

Molly

Poecilia sphenops

Sociable livebearer that prefers hard waterIntermediate

Lifespan: 3-5 years
Size: 2.5-4.5 in
Social needs: Social; keep 4+, more females than males
Time: 15 min daily; weekly water change
Cost: Medium
Diet: Omnivore, algae-heavy (flake, veg, pellet)

Overview

What is this animal?

Mollies are robust, sociable livebearers from the southern United States, Mexico and Central America.
Aquarium mollies are a mix of the shortfin molly (Poecilia sphenops), the sailfin mollies (P. latipinna and P. velifera) and hybrids between them, and they come in shortfin, sailfin and balloon forms and colours from black to dalmatian.
They are bulkier than guppies and platies and more demanding than the other common livebearers, so they suit a keeper ready to step up.

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They prefer hard, alkaline, mineral-rich water, and many populations tolerate or benefit from slightly brackish conditions.
Poor water chemistry, not poor hardiness, is the usual reason mollies fail in beginner tanks.

Temperament

Mollies are generally peaceful and social, best kept in groups and content in a calm community of similarly sized fish.
Males can be persistent in courtship, so keep a group of four or more with two or three females to every male, and give them space to reduce harassment.

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Larger males may occasionally show mild territoriality but rarely cause harm in a spacious tank.
They are prolific livebearers; mixed groups breed continuously, and adults eat fry unless dense planting or a breeding setup gives the young cover.

A good fit for

Keepers ready for a larger livebearer
Hard-water and lightly brackish setups
Algae-rich planted community tanks
Aquarists who can maintain stable parameters

Common mistakes to avoid

Keeping in soft or unstable water causing shimmies
Undersized tanks for their size and waste load
Protein-heavy, low-vegetable diets
Buying deformed balloon forms with health issues

Enclosure

Minimum 30 gallons, for a group of 4

Cannot be kept alone. The minimum is 4, and the space above is for that group.

Because they grow large and produce a lot of waste, mollies need at least 30 US gallons in a tank at least 36 in long, such as a standard 30-gallon (36 x 12 x 16 in), with strong filtration.
The giant sailfin molly (Poecilia velifera) needs far more: Austria's legal minimum for it is a 39 x 16 x 20 in tank (53 gallons), and its males' dorsal fins may not develop fully in a small or crowded tank.
Keep the temperature at 75-82°F in hard, alkaline water; adding aquarium salt or a marine mineral mix suits many strains.

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Provide open swimming length, planting, and algae-friendly surfaces.
Stable, well-buffered water is essential, as mollies are sensitive to ammonia and pH swings.
A tight lid contains them, and a mature, well-cycled filter is non-negotiable.

Check your enclosure

Supplies

Setup

Tank and stand
Filter
Heater and thermometer
Substrate, plants and decor
Liquid test kit
Dechlorinator and starter supplies

Ongoing

Quality tropical flake or micro-pellet
Spirulina or vegetable flake
Frozen brine shrimp and daphnia
Frozen or freeze-dried bloodworm
Veg clip for courgette and spinach

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Baby brine shrimp hatchery or frozen equivalent for fry
Fine net for removing leftovers

Safe substrate: Fine, smooth sand · Smooth, rounded fine gravel (not for loaches or sand-sifting cichlids; sand is better for corydoras) · Bare bottom (quarantine and grow-out only)

Work out the cost

Care routine

Daily

Check temperature and equipment
Feed
Head-count and look everyone over

Every few days

Top up evaporationevery 3 days

Weekly

Test the water
Water change and gravel vacuum
Wipe the front glass only

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Every few weeks

Trim and thin the plantsevery 2 weeks

Monthly

Check supplies and reagent dates
Replace or recharge chemical media, if you use any (skip it in a blackwater tank)
Review the fry plan
Rinse the filter media

Health review: Quarterly stocking and health review, every 3 months

Tools

Is your enclosure big enough?Check your measurements against the recommended size What will it cost?Setup, monthly and over its whole life, itemised Is this ammonia reading dangerous?Works out the toxic part from your pH and temperature

Care in depth

Diet

Omnivore, algae-heavy (flake, veg, pellet)

Mollies are omnivores with a strong herbivorous bias and need plenty of plant matter.
Offer a spirulina or vegetable flake base, blanched greens, and algae, supplemented with occasional frozen or live foods such as daphnia and bloodworms.

They graze constantly on biofilm and algae, so a mature tank helps.
Feed small portions two or three times daily and ensure adequate fibre, as a protein-heavy, low-veg diet commonly causes constipation, bloating, and digestive problems in balloon forms.

Health

Signs of good health, and the common problems

Mollies are prone to the so-called molly disease or shimmies, a quivering, near-stationary swimming that signals poor water quality, soft water, or mineral deficiency rather than a single pathogen.
Correcting to hard, stable, well-buffered parameters usually resolves it.

They also suffer ich, fungal, and bacterial infections after chilling or stress.
Balloon mollies in particular have deformed spines and compressed organs, predisposing them to swim-bladder and digestive problems, an ethical reason many keepers avoid that form.

Is this urgent?

6 warning signs and how fast to act, 3 of them right away

In a fish tank the patient is usually the water. Test before you treat: dosing a healthy tank for a disease it does not have makes things worse.

Go now

Any detectable ammonia or nitrite in a stocked tankLarge water change now with dechlorinated, temperature-matched water. Stop feeding. Test again in 12 hours. Both are directly toxic. How toxic the ammonia reading is depends on pH and temperature, so the same number can be harmless or lethal.

Now

Fish gasping at the surface with fast gill movement, or crowding the filter outflowIncrease surface movement now (gently in a betta or gourami tank, without creating a strong current), then test the water. Either the oxygen has crashed or the gills are being chemically burned. Both are hours-scale problems. Bettas, gouramis and corydoras rise to gulp air as part of normal breathing; the warning is constant gulping, fast gill movement, or the whole group hanging at the top.

Now

Sudden deaths, more than one fish in a dayTest ammonia, nitrite, nitrate and temperature before doing anything else. Do not add medication yet. Multiple simultaneous deaths usually point at the water or the equipment rather than a pathogen, so rule those out first.

Now

Today

Flashing: repeatedly scraping against decor or substrateTest the water, then look closely for parasites. Irritation is either chemical or parasitic; the test tells you which before you medicate.

Today

White salt-grain spots over the body and finsBegin the appropriate treatment today and check the temperature. The parasite's life cycle is temperature-driven, and it multiplies with every cycle you wait.

Today

Soon

Clamped fins, hiding, or refusing food for more than two daysTest the water and review recent changes. Usually the earliest visible sign of a parameter drifting.

A few days

Never add a new fish to a tank that is not fully cycled, and never treat blind. A test kit is cheaper than the fish.

This is general guidance for a healthy adult, not a diagnosis. An exotics vet who knows your animal always overrides it, and the time to find one is before you need one.

Bedding and substrate

3 safe, 4 with a caveat, 2 to avoid

Substrate in an aquarium is a filter as much as a floor. Most of the biology lives in and on it.

Safe

Fine, smooth sandCorrect for corydoras, kuhli loaches, eartheaters such as rams and acaras, and anything else that sifts or burrows. Barbels wear away on sharp gravel.

safe

Smooth, rounded fine gravel (not for loaches or sand-sifting cichlids; sand is better for corydoras)Rinse thoroughly before use. Grain size should be too large to swallow and too small to trap food. Kuhli loaches need sand to burrow into and rams and acaras sift mouthfuls of it; bronze and panda corydoras manage on smooth fine gravel, but sterbai corydoras need sand.

safe

Bare bottom (quarantine and grow-out only)Legitimate for quarantine and grow-out tanks. Easy to clean, but it holds far less biological filtration, and it is no long-term home for burrowing loaches or sand-sifters.

safe

With a caveat

Crushed coral or aragoniteDeliberately raises hardness and pH. Correct for rift-lake cichlids, wrong for soft-water species.

with a caveat

Aquasoil for planted tanksExcellent for plants, but most active soils soften the water and pull pH and KH down for months, and many leach ammonia for the first weeks. Right for soft-water species; wrong for hard-water animals (livebearers, rainbowfish, goldfish, and snails, whose shells erode in soft, acidic water) unless you test GH and KH and remineralise. Cycle around it; do not stock into it.

with a caveat

Very deep sand bedsAnaerobic pockets form below about 2 inches in still sand. Either keep it shallow or keep it stirred.

with a caveat

Painted or coated gravelUsually fine, but coatings chip. Test a handful in a jar for a week first.

with a caveat

Never

Sharp or angular gravel with bottom-dwellersWears down barbels and leads to infection in corydoras and loaches.

never

Anything containing metal, limestone of unknown origin, or shells in a soft-water tankSlowly alters chemistry in ways that are hard to diagnose later.

never

This is our general list for freshwater fish. We have not built one specific to Molly yet.

Top resources for more info

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