Description
The molly has a reputation as a delicate fish, and it is an unfair one born of a very specific misunderstanding. It is a robust livebearer of 8 to 10 cm, available in black, silver, dalmatian and dozens more varieties, that can live five years without giving you any trouble. If you know one thing.
It needs hard, alkaline water. This is not a preference, it is a requirement. Placed in the soft acidic water of an Amazon tank — which is where most end up, alongside neons and tetras — it sickens and dies, and nobody understands why. The fish was not delicate: it was in the wrong water.
A note on the name. Molly is a trade term covering several species — Poecilia sphenops, latipinna and velifera — and much of what is sold are hybrids between them. This profile describes the normal-dorsal fish; if you are after the one with the huge sail, that is the sailfin molly.
A welfare note: among the varieties you will find the balloon molly, with a shortened, rounded body. It is not a breed, it is a spinal deformity fixed by selective breeding, and it carries lifelong swimming and swim bladder problems. It is a personal choice, but worth knowing what you are buying.
Behavior
Peaceful and active, one of the most sociable fish you can keep. It uses the whole water column and does not hide.
The point that most affects its welfare is the same as in every livebearer: males harass females relentlessly. It is their main behavioural drive and cannot be removed, only spread out.
- Two or three females per male is the ratio that works.
- A lone pair exhausts the female and leaves her prone to illness.
- Males only generates hierarchy fights in small tanks.
It lives well with swordtail, platy, guppy, corydoras, bristlenose and nerite snails. They all share the key: hard, alkaline water. Real compatibility lies in that overlap of parameters, not in temperament.
Avoid housing it with cardinal tetra, rummy-nose, ram cichlid and generally any soft, acidic water fish: they will not fight, one of the two will simply always be uncomfortable. Also tiger barbs, which nip the fins of long-finned varieties.
With shrimp: adults coexist; fry disappear.
If you see your molly shivering on the spot without moving forward — known as shimmying — it is not a disease in itself: it is almost always water that is too soft, low temperature or a nitrite spike. Fix the water before medicating.
Habitat
Poecilia sphenops lives in rivers, coastal lagoons and estuaries of Mexico and Central America. The water is hard, alkaline and often somewhat saline, rich in vegetation with beds where algae grow abundantly.
Everything it needs in the aquarium follows from that. And so does a common misunderstanding: it does not need salt. Adding it is widely recommended, but in the wild it lives perfectly well in fresh water as long as it is hard and alkaline. Salt can help occasionally as a treatment, not as a permanent regime, and it is incompatible with most plants.
What you do need is real hardness. If your tap water is soft, the simple way to raise it is limestone in the decor or a calcareous substrate, not chemicals.
It also appreciates swimming space and some flow: it is an active open-water fish, not a corner dweller.
Details
🌊 Water parameters
🐠 Minimum tank size
Breeding
A livebearer. The female gives birth to fully formed fry able to swim, twenty to sixty per brood, every four to six weeks.
Nothing needs doing to make it breed: quite the opposite, the usual problem is controlling the population. And like the rest of the family, the female stores sperm for months, so one bought pregnant will keep giving birth with no male in sight.
The fry are independent but the adults hunt them. With dense-rooted floating plants a fair number survive; if you do not want the tank to fill up, that vegetation is precisely what you should leave out.
A practical point before buying: think about what you will do with the fry. A well-kept molly gives you several broods a year for years, and shops do not always take them.
Diet
Omnivorous with a far stronger herbivorous leaning than people assume. In the wild it spends the day rasping algae off stones and roots, and it keeps that behaviour in the aquarium: you will see your mollies picking at the glass and decor constantly.
- Staple: flake or granules high in plant content, or a livebearer-specific food.
- Plant matter: spirulina, nori or blanched courgette and peas, several times a week.
- Protein: frozen daphnia or brine shrimp, a couple of times a week. Enough, no more.
- Fry: the same food crushed.
The commonest mistake is feeding it like a carnivore. On a protein-only diet it develops digestive problems and constipation, which in this species shows up as the same shivering caused by unsuitable water.
And a welcome bonus: it genuinely helps with algae. It does not replace an otocinclus, but a group of mollies keeps the glass and decor noticeably cleaner.
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Plan your next aquarium
Is this fish right for you? Plan a suitable aquarium before setting it up.