By Hannah Nielsen
Why this guide exists
You searched "molly fish", and I'd bet money you've either lost mollies before or you're about to buy your first ones and every care sheet you've read says the same vague thing: "easy beginner fish, just keep the water clean." That advice gets mollies killed. The two things that actually decide whether your mollies thrive — your local water hardness and the salt myth — are the two things those pages gloss over.
I'm Hannah Nielsen. I photograph and write Tropical Fish Co's deep species guides from Cambridge, where my tap water is hard enough to fur up a kettle in a fortnight — and that hardness is precisely why mollies have always done well in my tanks while my soft-water friends struggle with them. This is the guide I'd hand someone who messaged me a photo of a molly rocking on the spot asking "what's wrong with it?" Nine times in ten, the answer is the water, not the fish.
A gold-black short-fin molly. The gold-and-black split, like the solid black of the hero fish above, is a line-bred colour selection of the same short-fin molly (Poecilia sphenops) — not a separate species. You're choosing a look, not a harder fish. Product photo · our warehouse.
Five things most UK molly guides never tell you
- Mollies are one of the only aquarium fish that prefer hard water. While most popular tropicals want soft, acidic conditions, mollies evolved in hard, mineral-rich, often coastal water and genuinely thrive at pH 7.0–8.5 and 15–35 °dGH [1]. For a UK keeper in a hard-water area, that flips the usual "fight your tap water" problem on its head.
- The sailfin molly can live in full seawater. Wild Poecilia latipinna is found across a salinity gradient from fresh inland springs to the open marine coast, which makes it one of the most salt-tolerant freshwater aquarium fish there is [3]. Tolerant — not dependent. It doesn't need salt to be healthy.
- The "shimmies" is a water-quality symptom, not a disease. A molly rocking side-to-side on the spot is telling you the water is too soft, too acidic or unstable for it [1]. Reach for a hardness test, not a medication bottle.
- A sailfin male's dorsal is a display flag, not decoration. The male raises that enormous sail-shaped dorsal fin in courtship and rivalry displays [3]. It's why a sailfin needs swimming length in the tank — a male with no room to display loses much of what makes him worth keeping.
- Mollies are vegetarians at heart. In the wild they graze enormous amounts of algae and plant material [1], which is why an all-protein flake diet leaves them bloated and short-lived. Greens aren't a treat for a molly — they're the staple.
Short-fin vs sailfin: which molly is right for you?
This is the first decision to make, because it sets your tank size. "Molly" covers two quite different fish in the trade, and a shop tank rarely tells you which you're looking at.
| What matters | Short-fin molly (P. sphenops) | Sailfin molly (P. latipinna / velifera) |
|---|---|---|
| Adult size | ~8–12 cm | Up to ~15 cm |
| Male's dorsal | Ordinary | Huge sail-shaped display fin |
| Tank length needed | 75 L+ | 110 L+, longer footprint |
| Price | Usually under a fiver | Often £10–£45 for premium strains |
| Best for | A first molly, smaller tanks | Keepers with a bigger tank wanting a showpiece |
| Care underneath | Identical hard-water needs | Identical hard-water needs |
If this is your first molly or your tank is on the smaller side, buy a short-fin. If you've got a long 110-litre-plus tank and want a centrepiece livebearer, the sailfin male is one of the most impressive fish in the hobby [2].
The strains we currently stock
Molly colour names are trade strains, not scientific varieties — black, gold, gold-black, silver, marble and the lyretail (lyra) forms are all line-bred looks layered onto the same two or three species. They share identical care. Here's a spread of our current short-fin and lyretail mollies, from everyday to a little fancier:
The cheaper strains are no less hardy than the premium ones — you're paying for rarer colour genetics and, with the sailfins, a bigger fish. If this is your first molly, start with a short-fin above and you'll do fine.
You will be told, possibly by a shop, that mollies "need" aquarium salt. They do not. Mollies need hard, alkaline, mineral-rich water — calcium, magnesium and carbonate hardness — not sodium chloride [1]. Wild sailfins tolerate brackish and even marine water [3], but in a freshwater community — especially a planted one — adding salt does more harm than good (it stresses plants and most tank mates). Get the hardness right and skip the salt.
Lyretail & sailfin strains
The lyretail ("lyra") forms trail an elegant, forked tail; the sailfins fly that flag of a dorsal. Same easy care, more drama in the tank:
A silver sailfin molly. That tall dorsal is the sailfin's signature — and the reason this fish needs a longer tank than a short-fin: the male raises it fully when he displays, and he needs the open length to do it. Product photo · our warehouse.
How many to buy, and the sex-ratio rule
Mollies are social, active livebearers — a lone pair looks lost and the single female gets pestered. Buy a group and skew it female: aim for one male to two or three females [4]. Males court constantly, and spreading that attention across several females keeps any one fish from being harassed to exhaustion [1].
A six-fish starter group — two males, four females — in a 75-litre tank is a balanced short-fin setup. Because mollies breed so readily [4], you almost never need to buy many; biology takes care of the rest. If you'd rather avoid a population explosion, keep an all-male group (all the colour, no fry).
The water question: why the UK is good molly country
Here's where mollies flip the usual UK fishkeeping headache. Most popular tropicals — tetras, rasboras, dwarf cichlids — want soft, acidic water, so keepers in hard-water areas are forever fighting their tap supply. Mollies are the exact opposite. They evolved in hard, mineral-rich, often coastal water and prefer it: pH 7.0–8.5 and 15–35 °dGH [1].
Thames Water — and most suppliers across the chalk-and-limestone belt of southern and eastern England — classify their supply as hard to very hard [5]. For a molly keeper, that tap is an asset, not an obstacle. Dechlorinate it and you've got near-perfect molly water for free.
Search your postcode on your water company's website (Thames Water, Severn Trent, Anglian, Affinity, etc.) for a hardness figure in mg/l CaCO₃ or °Clarke. If you're in a hard-water area, mollies are a brilliant choice straight from the tap. If you're in a soft-water area — much of Scotland, Wales, Cornwall, the Lake District — add crushed coral to your filter or use a remineraliser to bring hardness and pH up into the molly range before you stock.
Tank mates that actually work
Mollies are peaceful but boisterous, so the rule is: match the hard, alkaline water and pick tank mates that won't be bullied or out-compete them. These are companions I keep with mollies and trust:
- Platies — fellow hard-water livebearers with an identical care profile; a natural molly companion.
- Guppies — share the hard-water preference, though watch the bioload and the inevitable cross-tank fry explosion.
- Corydoras and other peaceful catfish — bottom-dwellers that occupy a different zone and share a tolerance for harder water.
- Peaceful tetras and rasboras that tolerate harder water — for mid-tank movement, provided your hardness sits in the overlap.
Avoid: fin-nippers (barbs can shred a sailfin's dorsal), aggressive cichlids, and any species that needs soft, acidic water — you can't give both groups the right chemistry in one tank.
Watch: a livebearer community tank in action
Transcript / what you're seeing (0:00–0:20): a planted aquarium with livebearers cruising the upper two-thirds of the tank, pausing to graze biofilm and algae off leaves and glass. Note how the fish stay out in the open and use the whole water column — that confident, busy behaviour is the tell-tale of healthy livebearers. Mollies that hang at the surface gasping, clamp their fins, or rock on the spot are almost always telling you the water is too soft, too acidic, or unstable for them.
Sexing and breeding
Sexing mollies is easy once they mature. Males are slimmer, often more colourful, and have a gonopodium — the anal fin modified into a narrow, rod-shaped organ used to fertilise the female internally. Females are larger, rounder-bellied, and have a normal fan-shaped anal fin; a gravid (pregnant) female shows a swollen belly and often a darker gravid patch.
Breeding needs almost nothing from you. Mollies are prolific livebearers — a female drops a brood of free-swimming fry roughly every four to six weeks and can store sperm to produce several broods from one mating [4]. The fry are large enough to eat crushed flake and baby brine shrimp immediately. In a planted community some fry survive in the cover; to raise a whole brood, move the fry to a separate grow-out tank or a breeding box.
A marble sailfin (Poecilia velifera). The mottled "marble" pattern is a line-bred look; underneath it's the same large, hard-water-loving sailfin that needs a long tank and plenty of greens. Product photo · our warehouse.
What they actually eat (greens, not just flake)
The commonest slow-burn molly mistake is feeding them like a carnivore. Mollies are grazing omnivores with a strong vegetarian bias [1], so build the diet around plants:
- Algae & biofilm — what they graze naturally; let some grow on the back glass and décor for them to pick at.
- A quality flake or pellet with vegetable content — the everyday staple.
- Algae / spirulina wafers — a reliable green supplement.
- Fresh blanched vegetables — courgette, cucumber, de-stalked spinach, shelled peas. Clip them in and remove leftovers the next day.
- Occasional protein — frozen daphnia or brine shrimp a couple of times a week keeps them in condition without bloating them.
What to look for when you buy (anywhere)
Whether you buy from us or anyone else, these are the welfare markers that separate a healthy molly from a problem one:
- Swimming normally and out in the open. A molly rocking side-to-side on the spot ("the shimmies") is showing water-quality stress [1] — a sign the shop's water may be too soft for them.
- A rounded but not pinched belly. Sunken bellies suggest starvation or internal issues; a long stringy white dropping trailing the fish is a parasite warning.
- Clean, complete fins — no white fuzz, no clamped or split tails. Especially check a sailfin's dorsal for nips.
- Ask which species and strain. A seller who knows whether they're selling a short-fin or a sailfin — and what water it's been kept in — is a seller who knows their stock.
Community & clubs
Mollies sit at the heart of the UK livebearer hobby, and the best place to learn strain genetics and meet breeders is the hobby itself, not a shop shelf:
- The British Livebearer Association (BLA) runs shows, auctions and a members' newsletter dedicated to guppies, mollies, platies and other livebearers — the single best UK resource for getting into molly keeping seriously (britishlivebearerassociation.co.uk).
- Local aquarist societies and the larger UK fishkeeping forums regularly have members offering home-bred molly strains, which is often how the nicer line-bred colour forms change hands.
When your mollies arrive: acclimation
Mollies are hardy once settled, but the one parameter they're genuinely fussy about is hardness and pH — a sudden swing between soft and hard water is the main thing that shocks them, and moving into hard UK tap water [5] can be a big jump from a shop's softer holding water:
- Float the sealed bag in your tank for 15–20 minutes to equalise temperature.
- Drip or top-up acclimate over 30–40 minutes — longer than for a guppy, because hardness and pH differences between bag and tank are the exact thing mollies dislike. Roughly double the bag volume before netting out.
- Net the fish into the tank rather than tipping the transport water in.
- Lights off for a couple of hours afterwards so they settle calmly.
- No feeding for the first 24 hours — they'll be fine, and it keeps water quality stable while they adjust.
Ready for more?
- Learn: our molly care guide for the full husbandry detail, plus the platy care guide and guppy care guide for the wider livebearer family.
- Compare: the honest balloon molly guide before you buy the short-body morph, the guppy buying guide, or the Variatus platy guide for another hard-water livebearer.
- Shop: browse the molly fish hub and the wider livebearers hub for everything that thrives in hard UK water.
