Molly
Poecilia sphenops
Sociable livebearer that prefers hard waterIntermediate
- Lifespan
- 3-5 years
- Size
- 6-12cm / 2.5-4.5in
- Social needs
- Social; keep in groups
- 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 Central America, available in shortfin, sailfin, and balloon forms and colours from black to dalmatian.
- They are larger and more demanding than other common livebearers, so they suit a keeper ready to step up.
- 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 a female-biased ratio and adequate space reduce harassment.
- 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 113 litres (30 US gallons) — for a group of 3
Cannot be kept alone. The minimum is 3, and the space above is for that group.
- Because they grow large and produce a lot of waste, mollies need at least 113 litres (30 gallons), more for sailfins, with strong filtration.
- Keep the temperature at 24-28C (75-82F) in hard, alkaline water; adding aquarium salt or a marine mineral mix suits many strains.
- 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.
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
- Baby brine shrimp hatchery or frozen equivalent for fry
- Fine net for removing leftovers
Safe substrate: Inert aquarium gravel · Fine, smooth sand · Aquasoil for planted tanks
Care routine
Daily
- Check temperature and equipment
- Feed
- Head-count and look everyone over
Weekly
- Top up evaporationevery 3 days
- Clean glass and clear algae
- Test the water
- Water change and gravel vacuum
Monthly
- Trim and thin the plantsevery 2 weeks
- Check supplies and reagent dates
- Review the fry plan
- Rinse the filter media
Less often
- Replace chemical filter media (Carbon/Purigen)every 2 months
Vet: Quarterly stocking and health review — every 3 months
Top resources for more info
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Diet
- 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
- 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 your enclosure big enough?
Is this urgent?
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
Today
Soon
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.
What it really costs
Setup, monthly, first year and across a whole life, itemised and with the electricity worked out from the actual wattage. Published figures for the same animal disagree by a factor of four, so the calculator shows ranges rather than picking one.
Bedding and substrate
Substrate in an aquarium is a filter as much as a floor. Most of the biology lives in and on it.
Safe
With a caveat
Never
This is our general list for freshwater fish. We have not built one specific to Livebearers yet.
My kit says 0.25 ppm. Is that dangerous?
A liquid kit measures total ammonia, but only the un-ionised fraction is really toxic, and that fraction is governed by pH and temperature — each whole pH unit multiplies it roughly tenfold. So the same reading can be nearly harmless in one tank and lethal in another; this works out which one you are in.
Compare with other animals
The same axes for every animal, side by side — the way the guides are written makes that a fair comparison rather than two articles.
Livebearers, side by side
Same axes, same care guides — the honest way to choose between types.
See all Livebearers side by side →
